Operational advances in short radius horizontal drilling have been achieved in conjunction with the placement of 23 drainholes in the Yates Field Unit, Pecos County, Texas. These advances include improvements in drilling of the curve section, application of gel squeezing technology to isolate the curve section, and drilling of the lateral section which are discussed for horizontal recompletions from existing vertical wellbores with 7 inch and 5-1/2 inch casing. In addition, the cost savings and performance enhancements realized from these operational advances are reviewed.
The most significant improvements have been realized through implementation of a short radius articulated mud motor system to replace rotary tool assemblies. The motor system has been optimized through refinement of motor and steering assembly specifications, and bit selection. The refined system has provided as much as a five-fold increase in penetration rates, a three-fold increase in lateral length, and a substantial improvement in directional control. The enhancements have also resulted in a reduction of over 60% in the cost per foot of drainhole length drilled.
Short radius horizontal completions have application where formation characteristics, well spacing, proration units or lease boundaries require a tight radius of curvature or limit drainhole length. The articulated short radius mud motor system can be applied from existing vertical cased wellbores allowing drainhole lengths approaching 1000 feet. This technique can be utilized without the investments required for "grass roots" medium radius wells. As a result, the technique is economically viable in a variety of geologic and operational settings, including the Yates Field Unit.
Short radius horizontal drilling has been successfully applied in the Yates Field Unit since 1986 providing a recompletion technique for wells with limited or no vertical recompletion potential. These techniques are more applicable than medium radius alternatives in the Yates Field due to proration and tract boundary constraints and in order to utilize existing wellbores. Also, the short radius curve limits the footage drilled through the reservoir's gas cap, minimizing lost circulation potential.
Yates Field horizontal drainholes produce from an oil column between a large, constant pressure gas cap and a weak aquifer in the San Andres formation. The San Andres formation is a highly Fractured dolomite with matrix porosity as high as 35% which is produced via gravity drainage. Facies variations within the dolomite, interbedded shales, and secondary calcite cementation cause localized reductions in permeability which contribute to reservoir heterogeneity and areas of less efficient drainage on the field's ten acre spacing. Horizontal drainholes allow recovery to be improved from vertical wells that were drilled in areas of reduced permeability by exposing additional productive formation.
The following sections detail procedures used to drill short radius horizontal wells in the Yates Field and improvements which have been made to develop the system into a viable workover alternative.
Drilling the curve is a critical part of any horizontal drainhole since it must end at both a predetermined depth and direction to maximize the length of lateral section within the desired target window. In Yates, a 10 foot vertical target window is used to minimize the vertical displacement of the drainhole in the oil column.