Successful Prediction of Sub–Basalt Velocity Profile Ahead of the Bit Helps in Taking Critical Drilling Decisions in a Deepwater, HPHT Well
Authors: Ranjan Prakash , Ashraf Ahmed , Singh Shefali , Chandrasekhar Srinivas , Sharma Shivam 
Published: November 2020

Over the years, drilling companies have had to go deeper into the earth and in more and more adverse sub-surface conditions to find new hydrocarbon reserves. Though there has been phenomenal progress in surface seismic acquisition, processing, and interpretation, drilling still involves uncertainty and associated financial, economic, and environmental risks. These risks, however, could be mitigated by using advanced tools and a holistic processing approach that integrates borehole and surface acquired data. (Hope, 1998). Vertical Seismic Profiling (VSP) is a unique wireline logging technique capable of looking ahead of the drilled Total Depth (TD). Using reflections coming from the interfaces below the TD, VSP lookahead can help in predicting the depth of the target ahead of the bit. The predicted acoustic impedance and, therefore, velocity is used in pore-pressure prediction, casing designing, and determining optimum drilling mud weight. VSP lookahead provides vital information about targets and hazards early enough to influence drilling decisions (Arroyo 2003).

In this paper, we describe how advancement in technology along with improved processing techniques helped in taking critical drilling decisions in a deepwater, High-Pressure, and High-Temperature (HPHT) exploratory well, XX. The well was drilled more than 4.4km below the sea-bed, on the eastern coast of India. Well, XX is in a clastic reservoir, with gas being the primary hydrocarbon. An HPHT environment existed with the expectation for the maximum temperature to reach above 350°F. Apart from the high-temperature regime, it was also anticipated that the well might encounter over-pressured zones, making drilling more challenging. Hence the prediction of pore pressure and mud weight of the section to be drilled was crucial.

Well was drilled in multiple runs with the acquisition of VSP at the end of each intermediate run to determine velocity profiles to be used for pore pressure prediction and optimization of the subsequent casing and drilling plans. VSP lookahead performed at the end of Run-2 helped in optimizing the casing design and determine the mud weight to be used for subsequent drilling. Run-2, VSP lookahead results showed a very good match with sonic and VSP velocities acquired in subsequent runs (Figure-3).

Upon reaching Run-4 TD, the well had just encountered the basalt. The client wanted to know the bottom of the basalt and pore pressure predictions for further drilling decisions. VSP lookahead was planned during Run-4 to determine the velocity profile ahead of the bit and hence predict Basalt bottom and pore-pressure profile. As the temperature in the well had reached above 350°F and standard VSP tool could not work under such extreme temperature conditions, extreme-environment digital VSP tool was used for data acquisition to ensure successful VSP lookahead. This advance VSP tool helped to perform VSP lookahead and imaging beneath high-impedance contrast Basalt, under HPHT scenario, which otherwise would have been very difficult.