Underbalanced Drilling Borehole Stability Evaluation and Implementation in Depleted Reservoirs, San Joaquin Field, Eastern Venezuela

Abstract

Abstract Drilling underbalanced is often expected to prevent formation damage, avoid lost circulation, and increase rate of penetration. However, it is also risky and may lead to borehole collapse due to lack of positive support provided by the borehole mud. Hence, its feasibility should be evaluated thoroughly through an accurate evaluation of in-situ stresses and a realistic estimation of formation rock mechanical properties. These two entities combined with the proposed borehole trajectory and mud weight design play an important role in avoiding borehole instability problems and achieving the field objectives. A study was conducted in San Joaquin field in Eastern Venezuela, to evaluate the feasibility of drilling underbalanced in highly depleted sands inter-layered with normally pressured shales. Data required for the study was obtained from five wells in the field (pressures, daily drilling reports, wireline logs, leak off tests and rock mechanical tests). A geo-stress model was developed, including pore pressure model, minimum and maximum horizontal stresses, and overburden stress. The in-situ horizontal stress directions were estimated from caliper logs, image logs and other field data. Formation rock mechanical properties were obtained using a program which utilizes the log data from the field and estimates the more representative static mechanical properties, which were then calibrated with the lab results. The combined geo-stress and mechanical properties models were calibrated with respect to drilling induced fractures and breakouts observed on the image logs. The calibrated models were used to estimate the required mud weights for drilling under various degrees of underbalance. Instabilities (breakouts) associated with each degree of underbalance were also quantified. The required mud weights to avoid shear and tensile failures, or the expected breakouts for a given mud weight, can be estimated from the generated contour plots for any borehole azimuth and inclination in the field. The results of the study were successfully implemented in the field, resulting in cost benefits of $1MM over five wells. Lost circulation was significantly avoided, and improvements in drilling rate as well as savings in rig days were achieved. The outcome of the study is expected to provide guidelines for underbalanced drilling for future wells in the area. Introduction Drilling deeper and in mature fields, requiring drilling through severely depleted reservoirs, is becoming increasingly lucrative due to the high energy demand. Depleted reservoirs are often interlayered with normally pressured zones. The combination offers a challenge to the drilling industry in terms of providing enough support to the normally or over pressured zones, and avoiding fluid losses in the depleted layers. Underbalanced drilling (UBD) is considered an alternative in such situations, where the mud weight is kept low enough to avoid exceeding the reduced fracture gradient in depleted sections. UBD is also expected to reduce formation damage and increase productivity. UBD has been deployed in many parts of the world, such as the North Sea,[1–3] continental Midwest,[4] South America,[5–7] Middle East,[8,9] and West Africa.[10] The reasons for such deployments range from increasing penetration rate in hard formations to reducing formation damage and avoiding mud losses or exceeding fracture gradient in depleted and narrow margin drilling cases. The purpose of the study was to conduct wellbore stability analysis to investigate the feasibility of underbalance drilling operations, and to recommend an underbalanced mud weight high enough to avoid excessive breakout (shear failure) and low enough to prevent mud losses. To achieve this, the rock mechanical properties and in-situ stresses were estimated from the available logs and drilling data. A geomechanical model was thus generated for the area. This paper describes the steps involved in generating the model starting from the log data, its calibration with respect to the image logs, caliper logs, and other observations made in drilling reports, and its application in predicting the required underbalance to avoid shear failure and mud losses. The recommendations made from this study were implemented in the field to drill several wells in the area resulting in increased penetration rates, avoidance of mud losses and substantial cost benefits.