Even mature reservoirs can enjoy the benefits of horizontal well completions through the use of ultrashort-radius, rotary steerable drilling systems.

Slimhole, rotary steerable, short-radius drilling equipment is one solution for widening the application of horizontal drilling to mature fields with relatively shallow reservoirs. The technology offers substantial cost savings over conventional mud motor technology and allows initiation of a lateral from an existing wellbore at a very tight radius. Developed during the early 1990s and applied in several mature US basins, the drilling system is being used to extend the life of Lekhwair field in Oman.

The field, in the northwestern portion of the Petroleum Development Oman (PDO) concession area, is about 90 miles (140 km) northwest of the main production center of Fahud in north Oman. Discovered in 1968 and brought on production in 1976, the field produces light, low-viscosity oil from two low-permeability (1 to 10 md) limestone reservoirs, the Lower Shuaiba and the Kharaib. The Lower Shuaiba is about 100 ft (30 m) thick, while the Kharaib is about 60 ft (18 m) thick, with 16 ft (5 m) of shale between the two intervals. Original oil in place was estimated at about 1.3 billion bbl (Willetts, et al., 1987). The field contains about 350 wells and has been developed by a line drive waterflood. Rapid production during the late 1970s resulted in rising gas-oil ratios from the Lower Shuaiba and, ultimately, to the initiation of the waterflood in the mid-1980s.

During the past several years PDO has drilled several horizontal and multilateral wells. The drilling of extremely long horizontal laterals resulted in significant production increases and supported plans for wells with openhole lengths of about 5,000 ft (1,500 m) and longer. As substitutes for multiple shorter lateral or vertical wells, these longer lateral wells were seen as cost-saving, production-accelerating opportunities. PDO also has seen the potential for drilling horizontal lateral injection wells to improve the waterflood's sweep efficiency.

The problem PDO encountered, said John C. Hunter, president of Terra Drilling Services LLC, was finding a way to drill these laterals accurately and cost-effectively into zones less than 35 ft (10 m) thick while avoiding surrounding shales, which can cause significant drilling problems, as well as relatively local, small-scale faults and fractures. "The intersection of the lateral with a fracture defeats the objective of drilling a horizontal producer or injector, since the water injection tends to preferentially enter the fracture rather than dissipate uniformly into the rock, which is needed for improved areal sweep. PDO believes that the chances of encountering a fracture are reduced if we can keep the entire curve and lateral in the target reservoir near the wellbore," Hunter said.

Terra Drilling Services specializes in drilling ultrashort-radius horizontal laterals with rotary steerable equipment. Their technology is being tested at the Lekhwair Field Well No. 365 injection well, which was drilled in early August. This trial, if satisfactory, will kick off a 2-year program. Paul Francis, Dynamic Value Delivery team leader with PDO in Oman, said, "This will be the first time outside the US for this technology and is part of PDO's drive to bring in technology expertise from around the world."

Although the first ultrashort-radius well to be drilled is an injector, Terra Drilling Services was brought into Oman to drill horizontal re-entries in existing producing or shut-in wells. In addition to Lekhwair, some of the probable candidates are wells in Barik field (Middle Gharif Sands) and Zauliyah field (Upper Gharif Sands). The Gharif Sands typically are 13 ft to 26 ft (4 m to 8 m) thick and separated by shales that can create major drilling problems. "By kicking off at the top of a given sand and keeping the entire curve and lateral within the sand, there is no need to drill through the shale and no need to run a liner for completion," Hunter said. "Furthermore, with rotary steerable horizontal drilling vs. a mud motor, it is possible to drill underbalanced using water, foam or air. This can be very important if there has already been pressure depletion of the reservoir." In the Lekhwair No. 365 well, two laterals of about 590 ft (180 m) will be drilled with radii of 35 ft (11 m). The lower one will target a 33-ft (10-m) portion of the Kharaib formation, while the upper lateral will target a similarly sized portion of the Lower Shuaiba.

The technology employed by Terra has its roots in a 1989 Amoco project initiated to develop a short-radius lateral drilling system that cost relatively little to manufacture and operate, could drill a consistent and predictable radius of curvature, was operable from a service rig with a power swivel and could work inside 4.5-in. casing. The system was commercialized, and since 1995, more than 100 wells have been drilled.

With Terra's system, bit rotation is derived from the power swivel with continuous pipe rotation throughout the curve and lateral drilling process. Directional control is a result of stabilizing the bit to continually point along a curved path using a deflection sleeve in the bottomhole assembly above the bit. Using composite or titanium drill pipe, ultrashort radii of 25 ft to 90 ft (8 m to 27 m) can be achieved. Because the system is purely mechanical and there are no mud motors or expensive electronics, overall well costs can be considerably lower. Drilling can be done with a service rig rather than a drilling rig.

Said Terra's Hunter, "This technology can be applied to the extension of field life in marginal fields anywhere. Drilling a short-radius curve within the target zone can minimize drilling and completion costs, so in most cases no liner is needed. Pumps can be set at the top of the target formation instead of several hundred feet uphole. The drilling can be performed with any fluid to minimize formation damage in depleted formations. It can be used in any marginal field where conventional horizontal drilling might not be feasible."

PDO's Francis is similarly optimistic about the application of ultrashort-radius rotary steerable drilling. "Our initial application was opening up thin interbedded fluvial sand channels, avoiding shales, possibly (drilling) underbalanced... to try to increase production initials. In the campaign for next year we will look at a series of criteria to rank vertical wells that are shut-in or low-volume producers as ultrashort-radius candidates. Ultrashort-radius sidetrack drilling has the potential to upgrade vertical stripper wells significantly."

Reference

Willetts, J.M. and Hogarth, R.A.M., "Lekhwair Pilot Waterflood, North Oman," SPE 15761, 1987.

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