Kamis, 14 Oktober 2010
Multiple Proppant Fracturing of Horizontal Wellbores
Throughout the early 90s, despite the introduction of horizontal well technology, production declined from BP’s Valhall field. In 1995 BP and Schlumberger introduced multiple proppant fracturing of the horizontal
wells. Since then, this technology has enabled Oil & NGL production to be increased by in excess of 50%.
Proppants are used to keep the walls of a fracture apart so that a conductive path to the wellbore is retained after hydraulic pumping has stopped. Placing the appropriate concentration and type of proppant in the fracture is critical to the success of fracturing treatments. Throughout the Valhall implementation the fracture spacing utilized has been determined by field calibrated production modeling. Optimum fracture dimensions and spacing along the horizontal wellbore are determined using an implicit reservoir simulator.
Results are used to determine economic indicators such as the Internal Rate of Return and the Net Present Value, depending on fracture spacing, fracture length, the cumulative risks and the effect of delayed production resulting from increased fracture frequency. This completion technique has proved a significant technical and economic success in the Valhall field.
However, the real success has been the synergy achieved between coiled tubing, perforation and fracturing operations that allows simultaneous operations (SIMOPS) to occur.
The Valhall field is an upper cretaceous, asymmetric chalk anticline that forms an overpressured, under saturated, oil reservoir located in the Norwegian North Sea. The field holds an estimated 813 MMSTB and 843 Bcf of gas. Current production reaches 87,000 BOPD from 43 wells on the main Drilling Platform (DP) and the bridge connected Wellhead Platform (WP).
The completion methodology utilized has been a developing strategy due to poor chalk stability that leads to formation influx and tubular collapse. Completion strategies have evolved from indirect, proppant fractured vertical wells, to gravel packed direct proppant fractured wells, then to horizontal blanket perforated wells. Initial productivity from the horizontal wellbores proved encouraging but by 1994 production had dropped by 20% due to high incidences of formation influx and terminal liner collapse.
Multiple proppant fractured completions were initiated in 1995 to couple the high productivity experienced from the horizontal wellbores with the longevity of the proppant fracture completions. Continued production gains have driven all
subsequent wells to be completed in this manner. In a typical 1,000 m horizontal section as many as 7 proppant fracture zones yield production rates as high as 10 - 12,000 STB/day compared to the 2 - 4,000 STB/day of earlier horizontal well completion techniques.
The first completion of this type in the field required 18 operating days to place 3 proppant fractures in a well drilled from the DP platform - an average of 6 days per zone. Subsequent development drilling was planned to take place from the WP platform. This requires a jackup rig to cantilever over the structure and drill in a 19 well slot arrangement. Exposure to such a high cost environment prompted an evolution in the way the completions are executed in order to significantly reduce both time and cost. Today each zone is completed in less than 2 days
by using specially developed techniques and tooling.
Once the liner and production tubing are set in place, all subsequent completion operations associated with the multiple fracture treatments do not require the services of a derrick and instead are performed by 23/8 inch coil tubing. The financial success of the WP development centered on making simultaneous drilling, production and completion operations a reality. The stimulation and coil tubing completion operation, which has come to be known as SIMOPS, is based on the main deck of the WP.
A total of 470 operating days have been saved from the jackup drilling rig schedule from 1996 through 1999 by performing these operations using SIMOPS.