The cost effective cutting of large intervals of continuous core in dolomitic formations requires optimization of existing coring practices. Coring costs can be reduced as much as 38% when innovative techniques are applied to the coring operation with regard to core barrel spacing, core barrel modification, and proper bit selection to include the use of polycrystalline diamond compact (PDC) bits and thermally stable diamond (TSD) bits. In addition, high percentage core recoveries and reduced problem time can be attained through teamwork and incentive arrangements between the operator and coring service company.
The South Justis Unit (Figure 1), located four miles East of Jal, in Lea County, New Mexico, is a new ARCO operated waterflood undergoing installation. The reservoir rock of the Unit includes the Blinebry - Tubb/Drinkard formations from approximately 5000' to 6000' which are of Lower Permian Age. Development of the Unit includes the drilling of water injection wells on 20 acre spacing to establish 40 acre five spot flood patterns. Prior to unitization, evaluation of the reservoir rock was necessary to accurately model water injection patterns and flood response. The estimated amount of core needed to develop a representative lithologic model was approximately 900 feet per well from four wells located within the Unit.
Two options for cutting the planned 3600' of core on the four wells included both conventional coring techniques and slimhole continuous coring techniques. Research into the coring options revealed that conventional coring was the better alternative for this field based on the prohibitive cost of slimhole continuous coring operations.
After determining that conventional coring was the most cost effective technique for coring the large intervals of Blinebry - Tubb/Drinkard formations, key issues that could make or break the project were assessed to include core jamming, insufficient core recovery, insufficient penetration rate, core barrel length, rig limitations, and lithology changes. These issues were addressed in an incentive turnkey coring contract which provided a means of sharing risk between operator and coring service company as well as incentives for reducing problem time and ultimate evaluation cost.