HORIZONTAL DRILLING TAPS MORE OIL IN THE MIDDLE EAST

Leo Aalund
Managing Editor-Technology

Keith Rappold
Drilling Editor

Horizontal wells have eliminated gas and water coning problems and increased oil production to improve ultimate recovery in several large oil fields in the Middle East.

In Saudi Arabia, Oman, and Egypt, horizontal wells have produced oil at rates 2-10 times that of offset vertical wells. As horizontal drilling becomes more common, the cost of horizontal wells will decrease to near that of vertical wells in the Middle East.

Saudi Arabian Oil Co. (Saudi Aramco) is moving forcefully into horizontal drilling. The company has prepared a special report on the specific opportunities its bountiful fields offer for this technology. Abstracts of this and recent Saudi Aramco technical papers were used to prepare this article.

Other companies in this rich oil producing region have found applications for horizontal drilling. A number of them revealed their experience and applications in papers prepared for presentation at the Society of Petroleum Engineers Middle East Oil Technical Conference & Exhibition in Bahrain, Apr. 3-6.

The technical advances contributing to horizontal well success include new polycrystalline diamond compact bits, conventional bits modified for high-speed motor drilling, steerable positive displacement motors, measurement while drilling, logging while drilling, innovative drilling fluids, and downhole adjustable stabilizers.

SAUDI ARABIA

In Saudi Arabia, 13 horizontal wells have been drilled in carbonates and 8 wells in sandstones, with another 8 wells in progress in the first quarter of this year.

The first two horizontal wells were completed in carbonate reservoirs offshore in 1991. Fig. 1 shows the range of productivity improvement expected for horizontal wells drilled in carbonate and sandstone reservoirs in Saudi Arabia.

Saudi Aramco says it has several major horizontal well applications:

Control of coning and edge water influx in relatively thin remaining oil column areas
Improvement of waterflood sweep efficiency
Production improvement in thin and tight reservoirs.

Each of these is also part of Saudi Aramco's plan for total reduction in development costs.

Over the next 10 years, Saudi Aramco's horizontal drilling program will make up half of the company's total drilling budget (Fig. 2).1 According to Saudi Aramco, a 5-10% improvement in oil recovery by application of horizontal drilling technology would translate into a 12.5-25 billion bbl increase in reserves in its fields. (Saudi Arabia has an estimated remaining proved reserve base of more than 257 billion st-tk bbl of crude, about 25% of the world's crude reserves.)1

Horizontal drilling technology, according to Saudi Aramco, has yielded numerous economic benefits to date for the company. It has:

Improved well capacity by 150-400%
Reduced the total number of wells by 30%
Reduced total drilling, flow line, and facilities costs by 20-25%
Reduced water and gas handling costs by up to 50%
Improved recovery of original oil in place by 5-10%
Deferred installation of artificial lift and gas compression facilities.

Most of Saudi Aramco horizontal drilling experience is derived from directional work on offshore platforms in giant Safaniya, Zuluf, Marjan, and Berri fields.

As of 1992, Saudi Aramco drilled successful horizontal wells in Berri, Zuluf, Abqaiq, and the giant Ghawar fields. Because of the relative immaturity of many of Saudi Aramco's fields, the company can apply horizontal technology very early in its fields.

WELL PLANS

The horizontal wells are typically long radius because of the long lateral displacements required.2

The typical casing program for the sandstone horizontal wells includes 9 5/8-in. intermediate casing set above the target reservoir at an inclination of 80 and at a depth of 6,000-7,800 ft MD followed by a 7-in. liner in the horizontal 8 1/2-in. hole set at 8,600-10,200 ft MD. The 8 1/2-in. horizontal section varies in length from 1,500 ft to 2,000 ft with inclination angles ranging from 89 to 91.

The liners are cemented in place because of the poorly consolidated sandstones, according to K.A. Al-Buraik and J.M. Pasnak with Saudi Aramco. 2

The typical casing program for offshore carbonate horizontal wells includes 9 5/8-in. casing set above the target reservoir at an inclination of 80 and at a depth of about 9,600 ft MD followed by an open hole completion.

The onshore carbonate horizontal wells typically have 9 5/8-in. casing set above the target reservoir at an inclination of 80 and at a depth of about 8,000 ft MD followed by an open hole completion.

The horizontal sections in the offshore carbonate fields have averaged about 2,400 ft, and the longest has reached 3,200 ft. Onshore, the horizontal sections range from 2,000 ft to 2,500 ft.3

Based on laboratory test data, non-damaging freshwater and salt-saturated polymer mud systems were selected for drilling the carbonate pay zones, and a special oil-based fluid was developed for drilling the sand reservoirs, according to A.M. Ezzat with Saudi Aramco.

A slightly acidic clean water or brine with, a water-wetting surfactant was typically used in the completions in the carbonate reservoirs. In the sandstone reservoirs, a clean water treated with anionic surfactant and mutual solvent was used as a perforating/completion fluid.3 4

All of the sandstone wells were perforated with 4 1/2-in. drill pipe-conveyed guns. The average perforated interval ranges from 560 ft to 1,500 ft, and the interval is shot in one trip.

The wells are perforated overbalanced because of the complication in killing the wells prior to pulling the guns out of the hole.

BERRI

Saudi Aramco first applied horizontal drilling technology in Berri field, which is located onshore and offshore along the western edge of the Persian Gulf. The field is 25 miles long by 12 miles wide.

Berri produced about 300,000 bo/d in 1991, and its cumulative production is more than 3 billion bbl of crude. Since 1973, the field has undergone secondary recovery operations in the form of waterfloods in the Hanifa and Hadriya reservoirs.

The carbonate nature of the reservoir improves hole stability and makes possible open hole horizontal completions, which produce at three to five times the rate of conventional vertical wells.

Berri field has become the prototype for horizontal drilling in carbonates in Saudi Arabia.

Saudi Aramco's first horizontal well, with a 2,303 ft drainhole, was completed in January 1991 in the Hadriya reservoir. The 8 1/2-in. horizontal section was drilled in 9 days.

The drilling fluid was freshwater-based with near neutral pH. A slightly acidic freshwater treated with a water-wetting surfactant was used for completion and well bore cleanup. The well's flow rate was triple that of vertical wells.

The second horizontal well was drilled and completed in April 1991 in the Hanifa reservoir. The 2,030 ft horizontal section was placed in the top 40 ft of the reservoir to drain the unswept oil and reduce water coning. The drilling fluid was a clay-free/seawater/polymer system chosen after a suite of compatibility tests on Hanifa cores.3

This well also had a several-fold increase in production compared with offset vertical wells. Fig. 3a shows the precise targeting of a well into oil above water in the complex carbonate Hanifa reservoir. The colors in the figure indicate reservoir variations.

ZULUF

Zuluf field was site of the first use of horizontal drilling in sandstones in Saudi Arabia (Fig. 4).

It is one of Saudi Arabia's northernmost fields, located about 35 miles offshore in about 118 ft of water. The field is 25 miles long by 9 miles wide and has eight multiple oil and gas bearing reservoirs.

Zuluf produced more than 900,000 bo/d in 1991, and its cumulative production is more than 2 billion bbl of oil.

The Khafji reservoir, with a strong water drive and a gas cap, is the main producing zone in Zuluf field. The formation is primarily a sandstone with shale stringers, shaly sand, coal/lignite/amber, and iron-rich shale/sand normally concentrated near the top of the reservoir.

The main sand is unconsolidated, highly porous, and permeable; thus, it can easily be washed out by pipe movement and excessive fluid velocities. The shale is somewhat water-sensitive. Oil-based muds provided the best well bore stability and lubricity in the high angle wells.3

Horizontal wells have exploited the Khafji thin sand and interbedded shale interval. Horizontal wells open more pay in the thin sand stringers than slant or vertical wells.

Several horizontal wells have quadrupled production compared to vertical wells in Zuluf. In the Khafji massive sand, horizontal wells target the upper portion of the sand for gas coning control on the crest and water coning control on the flanks (Fig. 3b).

Saudi Aramco plans to exploit the flank area Khafji stringer sands with horizontal drilling and to control gas and water coning selectively in the Khafji massive sand.

ABQAIQ

Abqaiq, an onshore field brought on line in 1946, is one of the most mature fields in Saudi Arabia. Some 37 miles long and 6 miles wide, it produced about 600,000 bo/d in 1991, and its cumulative production is about 8.5 billion bbl.

Abqaiq contains four hydrocarbon sequences, and most of the production to date has come from the Arab-D, which has undergone water injection since the 1950s.

The Arab-D waterflood will be optimized with horizontal injectors drilled in the unswept areas, Saudi Aramco says in its horizontal drilling report. Several producing wells have been drilled horizontally in an oil zone less than 50 ft thick at the top of the reservoir, resulting in higher oil production with a lower water cut.

Fig. 3c shows a typical lateral section in the top of a thin oil column in the Arab-D. The various colors indicate differing porosities, and blue signifies water.

The Hanifa sequence, which lies below the Arab-D, will be one of the world's first full scale waterfloods drilled from the outset with both horizontal injectors and producers. The flood will have a ring of horizontal injectors matched one on one with a ring of peripheral horizontal producers (Fig. 5).

The poor flow characteristics of the Hanifa make it well suited to the larger pay sections in horizontal wells. Saudi Aramco projects Abqaiq horizontal wells will have minimum water cut and will produce four to five times vertical well production, but with only a 50% increase in drilling costs.

GHAWAR

Onshore Ghawar field, about 174 miles long and 12 miles wide, is the largest producing oil field in the world. Ghawar produced about 4.25 million bo/d in 1991, and its cumulative production since discovery in 1948 is about 34 billion bbl.

Ghawar contains several oil and gas bearing carbonate reservoirs. The Arab-D is a very productive sequence in Ghawar; it has some aquifer support and has undergone water injection since the early 1960s.

Ghawar is the target for about 20% of Saudi Aramco's horizontal drilling. Half of the wells will be injectors and half producers.

The horizontal wells planned for the Arab-D will increase the efficiency of the waterflood. Horizontal injectors will provide line source distribution of water along the periphery of the field. Special injectors drilled just above a tar mat layer will pump water precisely into the flood. Another horizontal injector application is along the tight flank areas.

Saudi Aramco plans to drill horizontal producers in the unswept strata in the middle of the section or at the top of the section just ahead of the rising water level to recover oil not yet moved to a conventional well bore.

OMAN

Favorable results from horizontal wells have allowed Petroleum Development Oman (PDO) to increase its overall oil production rate without any significant increase in the rig fleet.

To date, more than 70 horizontal wells have been drilled and completed in fields in the PDO concession area, and PDO plans to drill 75 horizontal wells in 1993.5-8

In northern Oman in 1986, PDO drilled its first three horizontal drainholes using short radius technology and a rotary system. The technique was a success, but production was less than expected.

In 1990, horizontal wells were next drilled in Mussalim and Marmul North West fields, reservoirs previously thought to be uneconomic.5

NIMR

About 40% of PDO's horizontal wells have been drilled in Nimr oil field in the South Oman eastern flank. PDO plans to drill 35 horizontal wells in Nimr in 1993.5

Long term, PDO plans to replace vertical drain holes with half as many horizontal drain holes, according to S.M. Al-Rawahi, G.P. Small, and D.R. Glendinning with PDO.

Nimr field is one of the largest in the eastern flank and contains about 428 million st-tk cu m of oil in place. The heavy oil (21 gravity, 400 cp) is produced by 208 wells, 23 of which are horizontal. Current production is 63,000 bo/d.

According to the PDO authors, drilling a horizontal well in the eastern flank is cheaper today than drilling a vertical well was 3 years ago. Cost-effective casing design, drilling fluid optimization, and improved deviation and cementing techniques have reduced drilling and completion costs.

The immediate benefits of horizontal wells include increased recovery of heavy and viscous oil, delayed water breakthrough, improved productivity improvement factor, and reduced skin damage.

Reservoir studies have also shown a potential increase in the field recovery factor. The use of horizontal wells in Nimr field could increase ultimate recovery to 15-24%, up from 12%, and with only half the number of wells required for development with vertical wells.

Nimr field is underlain by an extensive aquifer, and rapid breakthrough is common because of high water mobility. Generally, vertical wells are completed in only the top 40% of the oil column. The poorly consolidated formation requires gravel packing.

PDO policy does not allow use of oil-based mud for environmental reasons. Therefore, the wells were drilled with water-based polymer muds. Partially hydrolyzed polyacrylamide and xanthan gum were used to stabilize the shale and encapsulate the cuttings.

The horizontal drainhole is completed with a predrilled or slotted liner for primary sand control. In contrast to a cemented and perforated completion, filter cake formed during drilling remains intact to form a potential barrier to flow. The calcium carbonate filter cake is removed by an acid wash during completion operations .6

The horizontal wells increase productivity by 1.3-9.3 times productivity of vertical wells, depending on pay thickness and vertical permeability. The initial horizontal well cost was 1.5 times the vertical well cost. Modified casing design and improvements in the mud system reduced drilling time to drop the cost to less than that for the old vertical wells.

SAFAH

Seven horizontal wells were completed in Safah field in Oman between October 1990 and February 1992. The incremental oil production from the horizontal wells will improve ultimate recovery, according to Hsiu-Kuo Chen with Occidental of Oman. 7

Safah field is 425 km west of Muscat and produces oil from a lime mudstone with an average permeability of 5 md. The field contains 650 million bbl of oil in place and 54 bcf of gas cap.

Prior to the first horizontal well, 114 vertical wells were drilled in the field. Some of these wells had early gas coning.

The first horizontal wells came on line at 1,500-1,800 bo/d, about five times the rate of the vertical wells. The drilling cost was 2.3-3.5 times the cost of the vertical wells.

Five additional horizontal wells were drilled, and the seven horizontal section lengths range from 1,985 ft to 4,718 ft.

Well performance indicates no direct correlation between initial well productivity index and length of horizontal section.

The long term productivity index of the wells depends on the gas/oil ratio and the index is typically about 10 times that of the vertical wells.

The gas/oil ratio and gas production of the horizontal wells in Safah field depend on the distance from the gasoil contact and the communication path between the gas cap and well bore. To avoid excessive gas production, the wells have to be placed as far away as is practical from the contact and should avoid penetrating any mapped faults.7

Six of the horizontal wells produce little or no water (less than 50 b/d). The seventh well has a higher water cut. The well appears to have penetrated a fault, allowing easier communication with the water. Because of the weak aquifer and large gas cap, future horizontal wells will be placed closer to the oil-water contact.

SAIH RAWL

Field production and simulation studies suggest the initial oil productivity and recovery per well of a 500-m horizontal producer in a thin zone is three to four times that of a vertical well, according to A. Al Zarafi with PDO.8 Encroachment of water to the horizontal well was found to be less of a problem than coning to vertical wells.

In 1991, a horizontal sidetrack was drilled and tested in the Shuaiba reservoir in Saih Rawl field in central Oman. Relative to other reservoirs in the field, development of the tight Shuaiba limestone has been unattractive because of rapid water breakthrough in vertical wells completed in the thin oil column.

The Saih Rawl Shuaiba reservoir, discovered in 1971, contains undersaturated light oil in a thin column (

The reservoir is penetrated by 28 wells, but only three are completed in the reservoir. Thus, there was much petrophysical data but little production or reservoir data for modeling the horizontal well. A radial simulation model was used to analyze the production performance of vertical producers for the prediction runs of the horizontal well.

Horizontal well performance was found to be principally dictated by oil column height, reservoir quality, and well spacing. The horizontal sidetrack was therefore drilled as close (2 m) to the reservoir top as practically possible.

The initial oil rate was 120 cu m/day, three times the productivity of vertical wells.

The reservoir simulations have found it economically attractive to use horizontal wells on a 500-m spacing to develop sections of the reservoir with a 20-m thick oil column. This plan may lead to 15 primary and 15 infill wells.

EGYPT

Two horizontal wells have been drilled and put on production in Egypt, increasing oil production and reducing water coning, according to E.A. El-Rafie with Khalda Petroleum Co.9

The Alam El-Buib 3E, or AEB 3E, reservoirs drained by the horizontal wells are in Hayat and Salam fields in the Western Desert.

Both reservoirs have very high vertical and horizontal permeability and severe water coning when produced by conventional unstimulated vertical wells.

The horizontal permeability in these wells averages 1,000-1,500 md and can reach as high as 3,000 md. The average vertical to horizontal permeability ratio is about 0.5:1.

The reservoirs range in thickness from 30 ft to 80 ft and are underlain by active aquifers.

Although the typical vertical well has a high productivity index of 20-40 b/d/psi, the production rates are restricted to prevent premature water coning.

In Hayat field, a single vertical well has drained about 2% of the original oil in place since 1987, and the well's water cut reached 85%.

In 1990, a horizontal well was drilled to tap the remaining oil, and the vertical well has since been recompleted uphole. The drilling of this first horizontal well was not a complete success because the formation was not completely uniform at a distance from the vertical part of the well, causing the horizontal section to drop near the oil-water contact.

Despite the horizontal section's coming close to the water, the actual productivity index for the horizontal well was 243 b/d/psi, compared with 17 b/d/psi for the vertical well.

The AEB 3E reservoir in Salam field was initially produced by several wells.

By mid-1990, several flank wells watered out, and some wells in the reservoir crest had high water cuts.

About 40% of the original oil in place had been recovered, and localized coning threatened ultimate field production.

A horizontal well was successfully drilled in the reservoir, and its horizontal section remained about 32 ft above the oil-water contact for its 1,100 ft length.

The productivity index for this well increased to about 380 b/d/psi, about 10 times greater than that for the vertical wells. The cost of drilling and completing the two horizontal wells was about 1.4 times that for the offset vertical wells.

REFERENCES

"Horizontal Drilling," Saudi Aramco publication, Fall 1992.
Al-Buraik, K.A., and Pasnak, J.M., "Horizontal Drilling in Saudi Arabian Oil Fields: Case Histories," SPE paper 25592, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
Ezzat, A.M., "Horizontal Drilling and Completion Fluids Design Criteria," SPE paper 25595, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
Menouar, H., and Huang, W.S., "Horizontal Well Design in Wafra Field, Ratawi, Oolite Reservoir," SPE paper 25597, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
Al-Rawahi, S.M., Small, C.P., and Glendinning, D.R., "Nimr Horizontal Drilling: A Case History," SPE paper 25591, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
Glendinning, D.R., Zwolle, S., and Vancura, J., "Low-Impairment Mud System for Drilling Horizontal Wells Through Clastic Reservoirs in a South Oman Oil Field," SPE paper 25545, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
Chen, Hsiu-Kuo, "Performance of Horizontal Wells, Safah Field, Oman," SPE paper 25568, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
Al Zarafi, A., "Breathing New Life Into a Thin Oil Column by Horizontal Drilling," SPE paper 25532, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.
9. El-Rafie, E.A., "Application of the First Two Horizontal Wells in Egypt," SPE paper 25534, presented at the SPE Middle East Oil Technical Conference & Exhibition, Bahrain, Apr. 3-6.