Drilling in Kutch Saurashtra Basin of Western offshore of India is characterized by slow ROP, frequent high torque and drag, wellbore instability and challenges while running casing. The exploratory well A is one of the deepest HPHT well to be drilled through hard, abrasive and thick massive layers of Basaltic formation (Deccan Trap) before reaching the Jurassic reservoir sands. A Geomechanics aided study was conducted and executed to improve drilling performance, optimize well condition and increase ROP to minimize rig days and cost of undesirable NPT.
A pre-drill 1D Mechanical Earth Model was prepared for well A based on offset wells for reservoir as well as for overburden rocks. Stable mud weight window was identified to optimize drilling mud weights. Motor with specially ordered abrasion resistant sleeves and compatible fixed cutter bit with innovative conical diamond elements (CDEs) were used to drill the harder Mesozoic formation (Jurassic to Cretaceous) inclusive of unexpected igneous intrusives, with remarkably good penetration rate. Torque & Drag reduction and tool wear were kept to minimal with optimal BHA design and drilling practices. LWD and Wireline conveyed pressure measurement tool was utilized for better understanding of reservoir complexity based on which mud weight was optimized in real time for the section.
Drilling parameters were optimized without compromising the ROP and 700 m of HPHT formation was drilled without stuck pipe and lost in hole incidents. More than 100% improvement in ROP in basaltic formation is observed compared to offset wells. Motor out-performed its operating hours in both runs (220 hrs. in first run & 370 hrs. in second run) without any failures.
Judicious selection of drill mud weight based on estimated stable mud weight window, resulted in improved wellbore condition in the reservoir as compared to offset wells, as indicated by LWD and wireline callipers. Improved borehole condition in reservoir reduced wireline "lost seals" pressure stations to 2 out of 6 attempted.
The CDE bit was field tested on PDM LWD BHA and drilled 813 m at an average ROP of 1.9 m/hr through the Mesozoic formation. The Volcanic dykes (Basaltic) had unconfined compressive strength as high as 33 kpsi making it vulnerable for conventional drill bits. Compared to offset wells drilled with roller cone bits, the CDE bit drilled eight fold increase in meterage at 40% higher ROP while providing good wellbore quality and came out with excellent dull condition (1-1-CT). The durability and high ROP of the CDE bit saved the operator about 15.7 rig-days of rig time and reduced cost per meter by 50%. This amounts to significant savings in monetary value.
Considering the high formations pressures, tectonic stresses and fractured nature of the formations LWD logging turned out as the best choice for acquiring virgin formation response and identification of zones of interest. Moreover, improved borehole condition and minimal drilling NPTs, reduced the operation risks and ensured achieving the well objectives. Effective operational procedures and constant communication with drilling team empowered them to take quick decisions regarding the well.