As fields mature and recovery factors reach technical limits, companies continue to search for ways to increase field longevity. The use of coiled tubing drilling (CTD) techniques in Alaska, USA1, Sharjah, UAE2, 3, 4, and other locations proves to be a promising access technology for re-entering old wellbores. BP America identified the Cleveland formation as a viable candidate on which to apply new technologies including CTD.
The Cleveland formation is an expansive gas reservoir located in the northeastern Texas Panhandle and western Oklahoma that was discovered in the late 1950's while searching for a deeper formation Fig. 1. Although initially developed with vertical wells in 640 acre spacing and traditional hydraulic fracture stimulation, vertical drilling was abandoned beginning in 1997 when the well spacing was downsized to 320 acres and replaced with horizontal wells.
The Cleveland is set at an average of 7300 ft. true vertical depth (TVD) and pay thicknesses vary from 20 to 80 ft. Porosity volume (PV) ranges from 4 to 15% with permeability from 0.003 to 0.015 millidarcy (md)5. Although the formation does not produce significant volumes, water saturation is approximately 30% and current reservoir pressures range from 950 to 1,800 psi.
Even in the best of times, development of the Cleveland has been difficult due to varied well performance, inefficient reservoir drainage, and marginal economics. Yet, to date it is estimated that only 25% of the original gas in place has been recovered from the Cleveland tight gas reservoir. Without a change in development, a significant volume of gas will be left undeveloped.7
BP proved that together with managed pressure techniques and CTD re-entries, it can provide cost-effective access for infill drilling activity in several areas of the world. Further development of this capability is the next step in opening up resources for tight gas reservoirs like the Cleveland.
In 2005, a 10 well CTD pilot campaign showed promising results. Building on the lessons learned from prior Alaska and Sharjah CTD campaigns in which old wellbores were re-entered using coiled tubing and then the existing casing was exited using whipstock systems. The wells were drilled overbalanced with conventional viscous mud systems. Shortly into drilling on the first well, it was obvious that managed pressure drilling techniques (MPD) were required to lower equivalent circulating density (ECD), minimize fluid losses, and prevent stuck pipe. The 2005 CTD pilot drilling operations were successful, but running the jointed pipe completion to bottom after drilling proved difficult. Of the 10 drilled wells, only three completions were successfully deployed. The promising drilling success in the asset using CTD enabled a series of tests in the front end loading (FEL) stage of 2006. As a result, CTD proved itself the most efficient method for infield reservoir exploitation after budget concerns eliminated rotary grass roots wells as an economic method of field development.