Drilling to the Extreme: The Micro-Coring Bit Concept

Abstract

Abstract The growing interest in deeply buried reservoirs increasingly leads operators into extreme drilling conditions characterized by high hydrostatic pressure (over 20.000 psi) and high temperatures (over 300 °F), combined with hard formations (high CCS– 40,000+ psi). Drilling in such environments requires specific adaptations of well design and in equipment selection. Additionally, these wells present multiple issues in well control, drilling and completion operations, and make the entire operation technically more complex and financially more risky. Referencing the specific example of drilling operations, the great depths and increased formation compaction leads to very low rates of penetration (ROP) and considerably extended operation time. These low ROPs equate to low depth of cut, and as a direct consequence the typical size of the drilled cuttings recovered at the shakers is extremely low (0.020 to 0.20 mm). This makes the cuttings irrelevant for the purpose of geological identification of the formations. This situation occurs with both impregnated diamond and PDC drill bits in these applications at low ROPs. If the downhole conditions (HPHT) exceed the MWD or LWD specifications, accessing critical geological information will necessarily require a challenging coring job, dramatically increasing the operation time and thus the cost of the well. In order to solve these problems, TOTAL S.A. and Diamant Drilling Services (DDS) developed a specific drill bit which generates micro-cores of formations during conventional drilling operations (from 10 to 40mm diameter depending upon application). This paper presents the different steps of the Micro-Coring Bit (MCB) development, starting with a description of the intial design concept. The experimental results of the drilling tests are presented. These tests validated the theoretical approach and led to the development of the first generation prototypes. Finally the first case histories (6″ PDC bit application for Total Indonesia, Tunu DX32 well – 8 ½″ Impreg bit application in exploratory environment) are reported, demonstrating the feasibility of the concept and highlighting the real benefits in terms of ROP improvement and cuttings quality.