Real-Time Interpretation of Supercharged Formations during Pore Pressure Monitoring

Abstract

Abstract The basic objective of real-time pore pressure (RTPP) services is to maintain the equivalent static density (ESD) and equivalent circulating density (ECD) within a desired mud weight (MW) window. This paper will present a new workflow to differentiate between supercharging in low-permeability, limited lateral extent sands, and genuine elevated pore pressure; ultimately preventing unwarranted MW increases that might either result in premature termination of the section or induce losses. In a typical workflow, log-derived models are used to compute the pore pressure (PP) in shales. These models are calibrated with drilling events and prompt formation pressure-while-drilling (FPWD) pretests in sands. However, in the Gulf of Mexico (GoM), sub seismic sand lenses are susceptible to supercharging; sometimes manifesting as an event gas peak. Moreover, using these gas events to determine supercharging have proven unreliable as they do not systematically occur. A novel workflow using time-lapse FPWD measurements, incorporating the acquisition environment, and the ability to circulate drilling mud at different flowrates to iteratively demonstrate the presence of supercharging has been developed. A common scenario is presented from a deep water well in the GoM in which both RTPP and FPWD services were run. As drilling progressed, the shale PP computed from the sonic logging-while-drilling tool was repeatedly validated with FPWD measurements. However, then a pretest conducted across an underlying sand showed PP value slightly higher than the computed shale PP. Based on conventional methodology, this condition would trigger a MW increase. Following regulatory requirements, ESD should be in a specified range above the confirmed PP. If the MW was increased further, the resulting ECD would be near the last casing shoe leak off test value, compromising wellbore integrity. If the flow rate was reduced to control the ECD, then wellbore cleaning would be compromised. A departure from the previously observed sand-shale PP equilibrium was unexpected and supercharging was suspected in this underlying sand. Clear evidence of supercharging was demonstrated by employing an iterative sequence of four repeated pretests with parameter adjustments. Experimental data were obtained and showed that no MW increase was required. Based on these results, the RTPP model was adjusted accordingly, and drilling continued without any problems to the planned section total depth. The presence of potentially supercharged sub seismic sand lenses complicates PP calibration. This new workflow is proposed to identify supercharging in these sands; thus, minimizing unwarranted MW increases, which could either result in premature termination of the section or induce losses. Either of these results could lead to operation cost overruns and extra casing and liner runs. The efficiency of the new workflow is demonstrated by the safe and successful drilling of a Deepwater prospect.