Diagnosing Near/Under Balance Conditions with Formation Pressure While Drilling Service
Authors: Mayans Aldrick Garcia , Paul Scott , Verma Vikhyat 
Published: November 2016

Formation Pressure While Drilling (FPWD) is regularly used in exploration and appraisal environments for drilling optimization purposes. The service is integrated into pore pressure prediction workflow and due to the uncertainty in the pore pressure, interpretation of the pretest acquired can sometimes be challenging. This is particularly relevant when the measured pressure is in a near or underbalance condition.

Near balance wellbore pressure is defined as formation pressure being marginally less than wellbore pressure. Underbalance wellbore pressure is when the formation pressure is equal or higher to the wellbore pressure. During wireline formation pressure testing the static logging environment contrasts with the dynamic drilling environment with respect to wellbore pressure, the latter can be affected by mud circulation amongst other parameters. An adequately overbalance condition with mud flow-on can change to underbalance condition when flow is static or changes are made to mud rheology. In exploration and appraisal drilling where pore pressure uncertainty is at its highest, the wellbore conditions can change from planned overbalance to an undesirable underbalance scenario if an unanticipated high pressure zone is encountered. Such a scenario may result in a well control situation.

Measuring formation pressure while drilling in near or underbalance situations poses challenges to the way formation pressure is measured. If the flowline pressure reads very close to, or is identical to wellbore pressure and is combined with limited data transmission rates then interpretation is difficult. The flowline pressure signature may look similar to a no seal or leaky seal pretest. Proper differentiation of the valid formation pressure and detecting a faulty seal are paramount to an accurate interpretation. Correct identification of these near/underbalance pretests is key to applying the right decision process and/or risk mitigation measures such as adjusting mud weight, drilling parameters or even casing depth.

Analysis is carried out on a large number of FPWD pretests. A workflow is devised to successfully analyze and diagnose in real-time, pretests acquired in near/underbalance conditions. This highlights different subjective factors that affect the formation pressure measurement in both scenarios and its sensitivity. A safe procedure is described to eliminate the factors which create uncertainty, and layout steps to prove the validity of the formation pressure measurement made in near/underbalance conditions in real-time. This workflow has been successfully implemented for FPWD services in the Gulf of Mexico and the results are used as examples in this paper to illustrate the concept.

This study enables understanding of the variables involved in interpretation of a near/underbalance pretest while the workflow assists in providing a systematic approach to diagnose the condition. The goal being proper mitigation of drilling events to avoid non-productive-time or an HSE event.