Drilling an exploratory well in the lower cretaceous formation of the Krishna Godavari (KG) Basin on the East coast of India poses a host of technical, logistical and environmental challenges. The technical demands were particularly acute when designing a drilling fluid to address the ultra-HTHP environment, the lack of offset data making it difficult to predict pore pressure, low seismic resolution and the highly environmentally sensitive ecosystem. While the technical demands downhole made oil-base (OBM) drilling fluids the system of choice from an operational perspective, the location of the block near a wildlife sanctuary, posed a host of logistical problems, in addition to the exorbitant costs associated with modifying the rig to comply with zero discharge restrictions required when using invert-emulsion drilling fluids. Thus, the operator embarked on a fast-track campaign to design a water-base (WBM) drilling fluid system that would not only meet the technical demands, but also address the environmental limitations.
This paper describes the design, execution and evaluation of the water-base drilling fluids employed on two HTHP wells in the KG Basin. This authors will discuss the effective use of a specially engineered KCl/Glycol system to drill 2000m of clay stone section in the 12¼ hole and the use of a low-colloid and contaminant-resistant water-base drilling fluid to reduce HTHP fluid loss values, improve shale inhibition, increase lubricity and reduce differential sticking potential for the formation. The first HTHP well in the block was drilled to 5061m with BHP of 11,800 psi and BHST of 410°F and presented a host of well problems due to the severe drilling environment. The nature of the formation coupled with the use of WBM limited the ability to achieve the logging objectives of the discovery well.
The high performance required of the WBM was compounded dramatically when planning the second HTHP well, which was to be drilled deviated to 5710m. Based on the results of the second HTHP well, the low-colloidal HTHT water-base mud system has proved to be an ideal alternative solution to the OBM. In addition, the authors will discuss the successful application of a mechanical earth model in predicting the pore pressure and helping WBM in achieving the target of drilling the deepest and most extreme environment well ever in India.
The Krishna-Godavari basin is located in the central part of the eastern passive continental margin of India. The structural grain of the basin is northeast-southwest. The basin contains thick sequence of sediments with several cycles of deposition ranging in age from Late Carboniferous to Holocene. The main reservoir is below Lower cretaceous unconformity with major lithology of sandstone intercalated with clay stone.
The lithology can be divided into three segments with the upper section comprising soft clay stone with hard limestone streaks. The intermediate section includes 2000m of homogeneous clay stone with limestone streaks followed by the 1000m of sandstone section with clay stones streaks between the lower cretaceous age. The main feature of the lithology is Lower cretaceous boundary at 4200m TVD which brings a sudden change in formation pressure and temperature.
A total of four wells were drilled in the exploratory campaign, two of which qualified as HTHP applications with operating conditions of 12000 psi and 410°F. The first of the two HTHP wells was vertical and drilled to 5061m; the second was a directional S-shape design with a maximum 29° inclination and drilled to 5600m.