Figure 1. Coiled tubing drilling was done with the fully automated Phoenix I. The 200-ton capacity rig has hydraulic controls for the injectors and the coil drum, coil life-monitoring systems and an integrated display for critical well parameters. (Images courtesy of Weatherford)
A groundbreaking five-well project in Algeria has successfully introduced and developed area-specific procedures for underbalanced coiled tubing drilling. The advance will help enhance production from one of the world’s largest oil deposits.

By tailoring a turnkey package of services to the mature Hassi Messaoud field’s reservoir complexity, low pressure and extremely hard, abrasive formation, Sonatrach has effectively applied a combined technology that was previously untried in the 1,000-plus well field.
The combination of underbalanced (UBD) and coiled tubing drilling technologies has proven to be a viable solution for successful drilling of horizontal re-entry wells in mature fields where the reservoir pressure is adequate to induce underbalanced conditions using a single-phase drilling fluid (flow drilling).

Project objectives
The project’s main objective was to enhance oil production from the field’s existing vertical wells by drilling horizontal slimhole re-entries. Underbalanced drilling technology was applied to provide better assessment of the producing zone, test-flow the reservoir while drilling and avoid formation damage. Coiled tubing drilling was selected due to the small production casing and the aggressive curve section buildup rate in the hard, abrasive formation.

Reduced drilling costs, increased ROP
The success of these combined technologies in the Hassi Messaoud field is attributed to the excellent balance achieved between drilling costs and improved productivity. Well productivity while drilling was significantly improved compared to offset wells. For example, when drilling the fourth well in the program a total of 12,540 bbl (1,994 cu m) of crude oil were produced in 20 days during UBD operations. Below 11,952 ft (3,643 m) measured depth, a continuous range of gas from 176.6 Mcf/d to 635.7 Mcf/d (5 Mcm/d to 18 Mcm/d) was produced. Figure 2 shows the oil and gas production behavior while drilling. This well is now one of the best producers in Hassi Messaoud field.

The productivity index while drilling (PIWD), calculated to quantify reservoir deliverability during the drilling phase, rapidly increased with vertical depth in the fourth well (Figure 3).
Over the five-well program, drilling costs were greatly reduced by eliminating pipe connections, increasing rate of penetration (ROP), prevention of conventional drilling problems such as fluid circulation loses and related differential sticking events, increasing bit life and the reducing time and distance drilled to reach the production zone.

The initial well costs were slightly over US $1.6 million; by the fifth well, expenditures had dropped to less than $1.2 million. These achievements were accomplished with zero lost time incidents over the five-well program. (Full results and a detailed examination of the drilling program are presented in SPE-106907-PP.)

Opportunity and challenges
Discovered in 1956, the Hassi Messaoud field was first developed with vertical wells. Many of these wells no longer produce and they form a large pool of candidates for remedial drilling.
The field covers an area of about 722 sq miles (2,000 sq km). Its primary production interval is the Cambrian-aged Ra formation at an average depth of 11,155 ft (3,400 m). The Ra is characterized by great variations in porosity, permeability and shale content. The target sandstone is extremely hard and abrasive, resulting in continuous measurement-while-drilling (MWD) failures due to bottomhole vibrations and very poor bit performance.

The complex reservoir is under-pressured, resulting in formation-damaging fluid losses when the first fractures are encountered. Mud loss also increases the risk of opening fractures that lead to premature water breakthrough.

State-of-the-art rig
The integrated project team assembled by Weatherford addressed five core services: coiled tubing drilling, controlled pressure drilling (CPD) services, directional drilling services, MWD tools and services and thru-tubing casing exits. Bits were supplied by several vendors based on Sonatrach performance records for the various areas.

Coiled tubing drilling was done with a newly designed, fit-for-purpose rig — the Phoenix I. The 200-ton capacity rig was equipped with 23¼8-in. coil fitted with internal capillary and e-lines, two hydraulic injectors, three Caterpillar engines, an SCR unit and two triplex mud pumps.

Fully automated, it featured hydraulic controls for the injectors and the coil drum, as well as coil life-monitoring systems and an integrated display for critical well parameters. Major rig loads were trailer mounted for fast moves in the desert conditions. The rig footprint was similar to a conventional rig of similar capacity and the rig could be rigged up on an existing cellar.

CPD services included equipment for pressure control, nitrogen generation, separation, storage and fluid cleaning. The flow parameters were measured and analyzed using a computer-based flow model.

Casing exit services included the anchor packer, whipstock and mills required for the casing exit. Directional drilling services involved mud motors and directional drilling services while MWD tools and services were comprised of MWD tools, spare parts and on-location support.

Dynamic duo: UBD and CTD
The drilling plan called for exiting the production casing and drilling in underbalanced conditions using native crude oil to minimize formation damage. A single, horizontal sidetrack in the Cambrian (predominantly in Ra) hole section was coil-tubing drilled to approximately 1,312 ft (400 m), depending on geological complexity.

There are three key aspects of the drilling plan: 1) Exiting the casing using a whipstock; 2) CPD build up rate (BUR) drilling and landing; and 3) CPD of a single horizontal/slant drain.
Three types of whipstocks were used to exiting the casing depending on the completion string in the candidate well. To exit through 41¼2-in. cemented liner, a packer-type whipstock was anchored in the well bore using a permanent, big-bore packer. A survey tool was then run to determine packer orientation and an orienting latch assembly was added to the bottomhole assembly (BHA) and oriented. The whipstock and starting mill were then run and latched into a matching profile inside the casing packer. With the assembly latched into the packer, the whipstock face was automatically oriented to the desired direction.

For casing exits through 41¼2-in. slotted, uncemented liner, the liner was first squeezed to place cement behind it. The cement was drilled out, and an anchor was run with whipstock, motor and gyro tool on coil to orient the whipstock.

Casing exits through 5-in. slotted, uncemented liner required an anchoring assembly to be run and set with cement prior to the wireline survey. The whipstock was then run conventionally on coiled tubing. Window milling was done in a single run using a diamond speed-mill and watermelon string mill.

The build section was kicked off with a steering gyro and 33¼4-in monocone/tri-cone bit on a low-RPM, high-torque motor. The adjustable kickoff angle of the motor was adjusted as required. To avoid hole problems in unconsolidated shale sections, multiphase flow was avoided during BUR drilling. The lateral was drilled using a 33¼4-in. impregnated bit and high-speed motor with a thruster added in the BHA.

Underbalanced conditions were designed to achieve several key objectives. It was important to maintain underbalanced conditions at all times while drilling the reservoir. Also, in the low-viscosity scenario, maintaining liquid velocity was critical to effective hole cleaning in the horizontal and vertical hole sections. Maintaining optimum liquid flow for downhole motors and obtaining minimum directional drilling parameters to reach the reservoir targets was also an important UBD consideration.

The combination of underbalanced and coiled tubing drilling technologies illustrates proven success for drilling horizontal re-entry wells in conditions where reservoir pressure is adequate to induce underbalanced conditions using a single phase drilling fluid, and where extremely low reservoir pressure requires the use of gasified fluids.

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