GEOL. CROAT. ZAGREB 1996 

Professional paper 

Some Experience with Drilling and Use of Horizontal Wells and 
Way of Their Application in Croatian Hydrocarbon Reservoirs 

Ivo STEINER' and Boris BURGER' PROCEEDINGS 

Key words: Horizontal wells, Geologica l structures, 
BHA. MWD, LWD, Well completion. 

Abstract 
In many hydrocarbon reservoirs hori zontal well paths enable (he 

more economical extraction of oil and gas than with venical wells in 
lhe same reservoirs. This is especially truc for cases of by-passed oil 
in partially depicted reservoirs. Successful design ,md drilling of hori­
zontal wells is made possible by detailed knowl edge orthe geologicul 
structure, together with up-l o-dale geo physical anc! olher data. The 
drilling and production of lhe firs! horizontal well in Croatia arc dis­
cussed and analysed. Methods for drilling and completion of horizon­
tal wells, together with a proposa l for thei r global utilization, are pre­
sented, 

1. INTRODUCTION 

Rccent technology facilitating the direction control 
of Ihe borehole path enables the positioning of a weJl 
from any starti ng point on the surface (or subsurface) to 
any dcsired point in the subsurface providing it is with­
in the technical capabilities of thc available drilling rig 
and drill tools (STEINER & BOSKOV STEINER. 
1994). Sueh wells consisl of: 

- a vertical section; 

- a build-up (curved) section, and 

- a horizontal or extended-reach section. 

The drilling or horizontal wells, i.e. those wells the 
final part of whose has a horizontal path, has become a 
routine operation for which the following arc prerequi­
si tes: 

a) a good knowledge of the geological structure of the 
drilled area, particularly of the target layer, its over­
laying and underlying strata (SOLOMON et aI., 
1994; WEST & BODNAR, 1994); 

b) 10 havc a corresponding bottom hole asscmbly 
(B HA) at disposal (AGAVANI el ai. , 1994; RAFIE 
el ai., 1986); 

I Faculty of Mining, Geology and Petroleum Engineering of the Uni­
versity of Zagreb, Pierot1ijeva 6, IIR-J 0000 Zagreb, Croatia. 

~ INA-Naftilplin. Subiccva 29, HR - 10000 Zagreb, CrO<llia. 

Kljucnc rijeci: vodoravne busoiine, geoloske strukture, 
sklop alatki na dnu busotine, telemetrija , geolo­
gometrija, opremanje busotina. 

Sazetak 
Voc!oravni kanali bllsotina omogllcavaju u mnogim IclistimH 

eKorlomic niji dohvat nafte i plina od uspravnih busotin<l, a osobito 
dohvat zaos tale nafte II djelomicno iscrpljcnim !czistima. Dobro poz­
navanjc gco ioskc grade, dopllnjeno svrhovi tOlTl primjenorn SUVfC ­

menih geofizickih i ostalih dostignllca, omogucava uspjesno projekti­
ranje i izvedbu vodoravnih busotina. R<lZmalraju sc isku stva izrade i 
uporabe prve vodoravne bllsotine u Hrv alskoj, IC se , llsporedbom sa 
svje lskim iskustvima, prcdlazu nacini izrade dalj njih tak vih busotina, 
njihovo opremanje i uporaba. 

c) to have corresponding measuremcnt while drilling 
(MWD), and logging while drilling (L WD) equip­
ment al disposal (ANON., 1993; TANGUY & 

ZOELLER, 1981; HANSEN & WH ITE, 1991); 

d) to have a well equipped drilling rig (if possible, with 
a top drive, a circulating elevator, etc.) for smooth 
drilling of the curved and horizontal part of the well 
(STEINER & BOSKOV STEINER, 1994); 

e) to have a well designed and well equipped system for 
drilling fluid circulation and control (a linear shale 
shaker, centrifuges, etc.) and an adequate drilling flu ­
id , as e.g. a polymer drilling fluid. Polymer fluids 
have good rheological properties, they thoroughly 
clean the borehole channel of rock cuttings, and they 
prevent settling of those cuttings. These fluids form a 
thin, tough filter cake on borehole walls which pro­
tects the productive zone from infiltration or the 
drilling fluid. This cake has a good lubricating prop­
erty and can be removed easily by a later realized 
nuid depression on the permeable and porous st rata 
(STEINER & BOSKOV STEINER, 1994). 

Naturally, selection of an adequate drilling fluid, 
pumping rates , control of torque and drag of the drill 
string and casing, blowout prevention and other factors 
affect the safe and effective drilling and these should 
not be neglected. 

The adequate completion of drilled horizontal wells 
is evcn more complex (AUSTIN el ai. , 1988; LESSI & 
SPREUX, 1988). Long-term and adequalely high pro­
duction of hydrocarbon f luids (oil and/or gas) from a 



horizontally drilled reservoir can be ensured by the 
right choicc and qualitative realization of well comple­
tion. This is especially true in the case where some by­
passed oil or gas arc targets (MOIIAMAD & SNI­
lDER, 1986). 

The most inexpensive and in some cases sufficiently 
productive methods of completion are an open hole, 
and landing a slotted uncemented liner (as in the Okoli 
horizontal well). However in heterogeneous layers, as is 
the case for most of the targets in Croatia, such solu­
tions may cause undesired mixing of oil, gas and/or 
water (KRILOY ct aI., 1996). Such mixing may mini­
mize or even cause cessation of the expected fluid pro­
duction. Sophisticated and relatively inexpensive com­
pletion methods of horizontal wells are available, and 
these may eliminate stich problems (DUDA & YOST, 
1995; BRADICK. 1993). 

Primarily, horizontal and ER (extended reach ::: 
nearly horizontal) wells arc drilled in reservoirs having: 

- low permeability and/or 

- problems with water or gas coning and/or 

- low thickness of the pay zone and/or 

- secondary permeability (fractures or fissures). 

2. PREREQUISITE GEOLOGICAL DATA 

Before producing a design for a horizontal well and 
defining it s radius or curvature, build-up rate, kick-off 
point, placement of tangent(s), etc. it is necessary to 
consolidate all the available data from vertical or devi­
ated wells in the region of the planned well. Geologists 
and geophysicists familiar with the area should be 
includcd into the team which is preparing and control­
ling the drilling and completion of a horizontal well. 
All availablc data on thc geological structure and geo­
physical surface measurement of the location have to be 
studied and any missing measurements (such as 3D 
seismic etc.) have to be obtained and analysed 
(SKOPEC, 1993; LAMUS PARA, 1992). In the case of 
the Okoli horizontal well the analysis was not based on 
a 3D seismic survey, but only on a 2D seismic study, 
together with detailed knowledge of the Okoli ficld and 
past experience o f similar scenarios. In addition to 
extensive knowledge of the expected gcological facies 
it is important to know thc rock stability and existing 
vertical restrictions inside the target reservoir. There are 
technical rcstrictions in obtaining large rock cuttings 
and core from the horizontal part or the well. Therefore 
cLlllings and cores from nearby vertical or less deviated 
wells have 10 be used for analyses (KRILOY et aI., 
1996). 

A lot of valuable data can be obtained in real-time 
by L \VD measurement, before seriolls formation dam­
age has begun. Comparison of these data with data 
obtaincd after the drilling is completed can give indica­
tions concerning the effective stimulation of future fluid 
production (LA YNE & YOST, 1994). 

Geologia Croatica 49n 

3. BOTTOM HOLE ASSEMBLY 

A number of particular too ls used for DBA (bottom 
hole assembly) exists, designed for deviated and/or [or 
straight sections of horizontal wclls. These arc subs. 
stabilizers, rDM (positive displacement motors) , non­
magnet ic drill collars, flexible drill pipes, etc. For those 
operators who drill only a few horizontal wells the best 
solution is to rent a service from one or the service 
companies specializing in horizon tal drilling or to rent 
only the necessary parts of an adequate BIIA from 
them. In this example a medium radius build-up rate of 
3°/10 m having a radius of250 m and a lateral deviation 
or 290 m were chosen. For such a job an accustomed 
drill string may be used without flexible drill pipe and 
other sophisticated tools. Computer models aiding the 
optimal choice of the BI-IA exist (BIRADES, 1986; 
KIKUCI-Il, 1993). 

Recently, horizontal wells are often drilled with rigs 
having coiled-tubing as a part of the drill string (WU & 
JUVKAM-WOLD, 1993; EDGE et aI., 1994). Even 
though voluminous and expensive cquipment is neces­
sary for drilling with coiled-tubing, it has the distinct 
advantage of shortening the time for round-trips which 
decreases the cost of horizontal drilling. Coiled-tubing 
and handling equ ipment can be rented at a reasonable 
price from companies producing them. 

4. MWD AND LWD 

In order to correctly drill a horizontal well it is 
essential Lo have data on the azimuth and dip angle of 
the well path . Collection of such dala in real-time 
enables the necessary corrections 10 be made by chang­
ing weight on the bit, bit rotation, bit face position etc. 
If data is obtained by mud pulses, which is the most 
widely used met hod, the necessary corrections on the 
hole bottom are limited, since there is only a one-way 
communication from the BI-TA toward the surface. In 
the application of mud pulsing signals there is a need 
for a homogenous, continuos fluid column in the drill 
string through which pulses are transported. 

If underbalanced drilling, with aerated drilling fluid 
or air drilling is applied, mud pulses cannot be trans­
ported Crom the BHA 10 the surface. Tn the case of infill 
wells toward by-passed oil and/or gas, it is often advis­
able to apply underbalanced drilling. Signals Crom the 
BRA can be transported by electromagnetic waves 
wherc the circuit is formed from the transmitter at the 
BI-IA through drilled rocks to the receiver on the sur­
face and backward through the drill string. Signals can 
bc sent in two directions, one carrying data from the 
BHA and the other carrying commands from the sur­
face allowing change to the parameters at the BHA. 

Electromagnetic MWD/LWD has to be taken seri­
ously as a convenient alternative in horizontal drilling 
for by-passed oil and/or gas and in drilling of re-entries 
in reservoirs with depleted pressure. 



Sicincr & Burgcr: Somc Expc ricnce with Drilling and Use of Ilorizonial Wells ... 

Diameter Diameter Planned Realized 
(mm) (inch) (m) (m) 

SOR.O 20 100 III 
339.7 133/R 1800 1898 

244.5 95/8 2360 2311 

177.8 7 2350-2860 2280-2442 

177.8 7 2243-2268 

137 5 2268-2892 

5. COMPLETION 

II' Ihere is any reason ror avoiding simple solutions 
such 3S an open hole completion or landing of a slotted 
liner without packing , stich as heterogeneous layers 
with variable oil , water and gas saturation, various per­
meability, pressure differences in drilled rocks in the 
horizontal part of the well , loose sand ctc. it is advi s­
able to apply designs with inflatable packers, partial 
cementing, grave l packing and/or with slotted liner hav­
ing s lidin g ports. For more effect ive drainage of a 
drilled reservoir a multi-branch completion may be an 
effective solution (STEINER & BOSKOV-STEINER. 
1994). 

As the purchase and landing of liners with existing 
sophi st icated completion methods is a very expensive 
and complex venture, it should be thoroughly analysed 
whether the invested amount o/" money can be covered 
by the value of increased production. 

6. OKOLI WELL - THE FIRST HORIZONTAL 
WELL IN CROATIA 

The Okoli field is a gas and gas condensate produc­
ing field silUated in the Sava trench, the most Southern 
part of the Pannonian basin. Tn Tertiary sed iments there 
are three productive reservoir-series: A, Band C. 

The A-series is closest to the surface. It was depict­
ed until a pressure grad ient of .44 bar/l 0 m was 
reached. 11 is now serv in g as a subsurface gas store 
(SGS ). When it is filled with stored gas the pressure 
gradient is elevated to .94 bar/IO m. When the vertical 
part of the horizontal well was dril led through thi s 
se ri es the pressure gradient was at the highest level. 

The B-se ries is in production and its pressure gradi­
ent varies between .31-.33 bar/l 0 Ill. 

The C-series reservoir, which was the target of the 
hori zontal well, is an anticline consisting of a number 
of thin , sandstone deposits characterised by low-penne­
abi lity and interlayered with non-permeable shale. The 
horizontal well path is si tuated on the top o/" the anti­
cline with its lateral part following the crest o/" the elon­
gated anticline. The pressure gradient in the C-series 
varies between .45-.90 bar/lO m. 

The aim of the first horizontal well in Croatia was to 
inc rease gas production from a low permeability, thin 

Remark 

covered SGS" 
many differ. stuck - pipe 

stuck-liner 

101 

Table 1 Planned and realized casing 
program Okoli horiWlllal 
well. • SGS = subsu rfac e gas 
storage. 

layer reservoir (so called sandwich st ru ct ure) and to 
prove whether a horizontal well is able to produce al 
least 1.5 times the volume of gas per day than thal of a 
vertical well stimulated by hydraulic fracturing in the 
same reservoir. Therefore a thorough study was under­
taken with a calculated four year period for return of 
the investment. 

Taking into consideration the available tools , equip­
ment and fin ancial resource, the TNA -Naflaplin compa­
ny decided to drill a hor izontal well into the Okoli gas 
bearing C-series from a kick-otT-point at 1,925 m. It 
was planned to case the lateral section (of about 500 Ill) 
with a slotted liner or 7 inch diameter and 10 cover 110n­
stable reservoir rock with that liner. The Okoli horizon­
tal well casing program is shown in Table I. 

Rea lization of the curved and horizontal part of thi s 
first horizontal well in Croatia was entrusted 10 an 
experienced in te rnational servi ce company. The aim 
was to include a partner able to offer an integrated ser­
vice which would help in drilling, well completion and 
commission ing of the horizontal well. 

Due to low pressure gradients the underbalanced 
drilling method was considered. However operating 
company did not have the necessary equipment. Rent­
ing of such equipment and expense of purchasing nec­
essary material meant that the expected financial resuil 
from well production would be lower than the financial 
inves tment req uired and this method was therefore 
abandoned. 

The gas reservoir of the B -series is located in the 
build-up section of the well. Sillee the pressure gradient 
in this series is low (.31-.33 bar/lO 111) and the polymer 
drilling fluid had a density of 1.050 kg/m ' . il was 
expected to get stuck pipe due to dil1erential pressure. 
But since the filter cake was f ~ttty and slippery this was 
expected to be miti ga ted. However due to frequent 
sticking pipe it was decided to switch from a slidin g 
mode to rotary mode, which caused the build -up rate 
(BUR) to be losl. Drilling was stopped and it was 
decided to co ntinue with a small er bit diameter and 
with a more flexible drill string, in order to retrieve 
back the desired BUR. At a depth of 2,311 m a 9 5/8 
inch diameter casing string was landed and cemented. 

When the final length of the hole at 2,900 m was 
achi eved , an attempt had been made to land a slotted 
liner of 7 inch diameter. This li ner was equipped with a 
wash pipe in case the liner became stuck so that, by cir-



302 

culation and by applying the driller's method, the liner 
may be freed. The sloHed liner was also equipped with 
spiral, rigid blade centralizers. During running of the 
liner it became stuck when its casing shoe was at 2,472 
m (161 m beyond the previous casing). The wash pipe 
failed. A top drive, a power swivel and/or circulating 
elevator were not available. By pulling it, the liner was 
made free, and when pulled for about one pipe stand, at 
a length of 2,442 m it became stuck again (a centralizer 
stuck in the shoe of the 9 5/8 inch casing string). This 
problem was solved by cutting the 7 inch liner at 2,280 
Ill, 3 1 m above the shoe of the 9 5/8 inch diamctcr cas­
ing. The free part of the 7 inch liner was pulled out of 
the hole. After drilling through the bottom assembly of 
the 7 inch pipe the hole was completely cleaned, fol­
lowing which a 5 inch slotted liner wilh a wash pipe at 
the shoe and no cent ralizers, was run Lo the well bot-
10m. A liner hanger, prepared for the 7 inch liner, was 
adapted with a 7x5 inch joint and was used for fixing 
the 5 inch liner to the previous casing stri ng. 

The well was finished according to the designed tra­
jectory and generally completed accord ing to the 
designed casing program, with cxeeption of the substi ­
tution of a 5 inch liner instead of the designed 7 inch 
one. It was accomplished according to the predicted 
time schedule, except of an additional len days spent 
for fishing job. 

According to laboratory tests with core from a simi ­
lar reservoir (KRILOY et aL, 1996) it is es timated that 
a formation damage of 33% of the original permeability 
exists, caused by the applied polymer drilling fluid and 
a synthetic formation water. This test may bc repeated 
sincc core exists from the same reservoir and formation 
water from it may be collected. 

Production equipment with a permanent packer and 
a 3 1/2-inch tubing was run and sat isfactory gas produc­
tion has been achieved. The horizontal well has attained 
following production: 

a) 500% higher production than that of an average ver­
tical , unstimulated wel! from the same reservoir; 

b) 180% higher production than that from a vertical 
well in the samc reservoir which was stimulated by 
hydraulic rracturing. 

Purchase of equ ipmcnt and tools needed for the lat­
e ral interval or the well requi red approximately 6% of 
the total cost of the well. Service cos ts (special tools 
and equipment, MWD and their handling) have 
amounted to about 8% of the total drilling costs. For 
various materials 45 % o/" that cost was needed and 55% 
was paid to the service company. The horizontal weJ[ 
was in toud 44% more expensive than an average verti­
cal we ll or the same length. The cost per meter of the 
drilled horizontal well was approx im atcly 900 USD. 

In the horizontal part of the borehole no classical 
wcll logging was done as it were no t needed and the 
equipment and tools required such as carriers and joints 
with side door opening for entrance of the cable were 
unavailable. 

Geologiil CrOJtiCil 49/2 

Workover in the hori zontal part of the we!! could 
not be performed either as there were no adequate 
coiled tubing and necessary lools available. Application 
of ECP-s (external casing packers) was not planned 
because they arc too expensive in comparison with cal­
culated value of producable gas and because the equip­
ment for activating such packers was unavailable. Con­
sequently , no stimulation job can be performed in this 
borehole because no section surrounding the borehole 
can be isolated. 

7. CONCLUSIONS 

Globally the appl ication of horizontal wells for 
increasing the hydrocarbon fluid production from other­
wise inaccess ible or poorly accessible reservoirs has 
become an everyday practice in the world, and thou ­
sands of such welJs are drilled annually. In the search 
for by-passed oil and/or gas and for their production 
horizontal wells are the most effective means. In Croa­
tia we can expect a reasonable quantity of by-passed oil 
and/or gas in a number of previoLlsly discovercd and 
non -rationall y depleted reservoirs as wei! as in the 
vicinity of more recently discovered ones. Th ere is a 
necd to study each of the existing reservo irs and estab­
lish whether horizontal wells would help finding exist­
ing by-passed oil and/or gas and increasing the oil and 
gas production. 

Without a decision to drill significantly more hori­
zonta l wells in a short period there is no reason to pur­
chase or produce special equipment and tools for hori­
zontal and/or ER-drilling. It is more reasonablc to hire 
specia lized serv icc companies with good references 
who arc able to offer a reasonable price of serv Ice In 
fair competition. 

Acknowledgment 

This paper is based on work sponsored by the Croa­
tian-Am eri can Joint Fund in cooperation with the US 
Deparlment or Energy under project JF No. 278. 

8. REFERENCES 

AGAVANI, M., RAHMAN, S.S. & MAIDLA, E.E. 
(1994): A new approach to selecting optimum bot ­
tom hole assembly configuration for any given well 
trajectory.- SPE Asia Pacific Oil and Gas Confer­
ence. SPE paper 28774,9 p., Melbourne. 

ANON. ( 1993): State od the art in MWD.- 2ndEdit., 
Inti' MWD Society. Houston, 32 p. 

AUSTIN, c.. ZIMMERMAN. c., SULLAWAY. B. & 
SABINS, r. (1988): Fundamentals of horizon ta l 
well completion. - Drilling, May-June, 28-31. 

BIRADES. M. (1986): ORPHEE 3D: Static and dyna­
mic threedimensional BHA computer models.- 61 til 



Steiner & 13urger: Some Experience wi th Drilling and Use of Horizontal Wells ... 

Annual Tech. Coni". and Exhib. of the SPE, SPE 
paper 15466, New Orleans, 12 p. 

BRADlCK, B. ( 1993) : Vizszintes kutkikepzesi modsz­
erek.- KoeolaJ es foeldgaz, 25(125)/7, 207-210. 

DUDA , J.R. & YOST, A.B. (1995): DOE/Fossil ener­
gy's drilling, completion and stimulat ion R&D : A 
Techno logies/Products Overview.- SPE Eastern 
Regional Cant". and Exhib" SPE paper 30993, Mor­
gantown, 16 p. 

EDGE, E., !lRINKHORST, J. , VOELKER , [-I., !lUR­
GE, P. & EVEN, R.t. (1994): Funhcr advances in 
coilcd-Lurbing drill ing.- EUROPEC, SPE paper 
28866, London, 9 p. 

HA NS EN, R.R. & WHITE, J. (1991): reatures in log­
g in g-w hilc-d rilling (LWD) in horizontal wells.­
SPE/JADC Dril ling Conkrcnce, SPE paper 21989, 
Amsterdam , 9 p. 

KIKUCH[, S. (1993): 2D & 3D planning for horizontal 
wells.- SPE Middle East Oil Tech. ConI. and 
Exhib., !lahrain , SPE paper 25647, 15 p. 

KR[LOV, Z., WOJTANOWICZ, A.K. & TOM[C, M. 
(1996) : Formation damage in horizonta l wells 
caused by polymcr mud. - SPE Formation Damage 
Symposiulll, SPE paper 31135, Lafayette, 16 p. 

LAMUS PARA , A.U. (1992): Operational geological 
experiences of hori zontal wclls.- Sccond Latin 

American Petro l. Eng, Coni". (II LAPEC) of the 
SPE, SPE paper 23654, Caracas, l i p. 

LAYNE, A.W. & YOST, A.B. ( 1994): Development of 
advanccd drilling, completion and stimulation sys­
tems for minimum formation damage and improved 
e lTicicncy: A Program Overview. - SPE IntI ' Symp. 
on form . Dam., SPE paper 27353, La f"ayette, 7 p. 

LESS[, J. & SPREUX, A. (1988): Complet ion oChori­
zon tal clrainho les .- S PE Meeting on Petrol. Eng., 
SPE pape r 17572, Tianjin, 10 p. 

303 

MOHAMAD, A.Y. & SNlJDER, G.t. (1986): Identifi ­
cation of bypassed oil usmg Gamma-Ray 
spectrometty tool in old wclls. - SPE Offshore South 
East Coni". and Exhib. , SPE paper 14604, Singapore, 

10 p. 

R AFIE, 5., HO, H.-S. & CHANDRE, U. (1986): Appli­
cation of a DHA analysis program in dircction 
drill ing. - SPE/IADC Drill ing Confcrence, SPE 

paper 14765, Dallas, 10 p. 

SKOPEC, R.A. (1993): Characterization in reservo irs 
targeted for horizontal drilling.- JOLlr. Pel. Tcch., 
1168-1176, SPE paper 22709. 

SOLOMON, S.T., ROSS, K.C. , BURTON, R.C. & 
WELBORN, J.E. (1994): Multidiseiplined approach 
to designi ng targcts for horizontal wells. - Jour. Pet. 
Tech., 143-149, SPE paper 25506 . 

STEINER, I. & BOSKOV STEINER, Z. (1994): 
Tchnologija vodoravnoga buscnja. - Barbat, Zagreb, 
170 p. 

TANGUY, D.R. & ZOELLER , WA (1981): Applica­
tions of measuremcn t whi le drilling. - 56111 Annua l 
Fall Techn. ConL and Exhib. of the SPE, SPE paper 
10324, San Antonio, 16 p. 

WEST, c.c. & BODNAR, DA (1994): Reviewal' the 
geological environment for horizontal we lls. - Sec­
ond Arch ie Colloqu ium, SPE paper 23544, Ho us­
ton, 10 p. 

WU, J. & JUVKAM-WOLD, H.C. (1993): Drilling and 
completing horizonta l wells with coiled tubing. - 681h 

Annual Tech. Conf. and Exhib. , SPE paper 26336, 
Houston, 17 p. 



30-1 Geologia OWl1 ic;, 49J2