Abstract

This paper describes an experimental investigation of formation damage during underbalanced drilling (UBD) conditions in a fractured carbonate core sample. A major portion of this study has concentrated on problems, which are often associated with UBD and the development of a detailed protocol for proper design and execution of an UBD program. Formation damage effects have been studied, which may occur even if the underbalanced pressure condition is maintained 100% of the time during drilling operation. One major concern to formation damage during UBD operations is the loss of the underbalanced pressure condition. Hence, it becomes vital to evaluate the sensitivity of the formation to the effect of an overbalanced pulse situation. The paper investigates the effect of short pulse overbalance pressure during underbalanced conditions in a fractured chalk core sample. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Both overbalance and underbalanced tests are conducted with four UBD drilling fluids. Core testing is including the initial permeability measurement and return permeability after two different conditions of pressure (underbalanced and overbalance). Then the procedure is followed by applying a differential pressure on the core samples to mimic the drawdown effect to present capacity of the return permeability. In both UBD and short pulse OBP four mud formulation are used which are, lab oil, Brine (3% KCL), Based mud (Bentointe with XC polymer) and fresh water. The return permeability measurements show that a lab oil system performed fairly well during UBD and short OB conditions. The results indicate that short overbalance pressure provides significant reduction in permeability of the fractured carbonate formations. In most tests, even application of a high drawdown pressure during production cannot restore the initial permeability more than 40%.

Introduction

The presence of high permeability features in a formation, such as large naturally occurring fractures or an extensive interconnected vugular porosity system; represent a significant challenge for overbalanced drilling operations with respect to rapid and deep invasion and significant, often permanent permeability impairment. In most situations, the high permeability of the fractures and vug systems act as conduits to feed gas or oil from a tight producing source matrix to the wellbore for production. This being the case, the preservation of the high permeability fractures and vugs is of prime importance.

It is often very difficult to assess the in-situ-size distribution of fractures (although mico-resistivity logging tools can provide some indications) and many reservoirs may exhibit a wide range of potential fracture apertures, making the design of an effective overbalanced bridging system in such a situation difficult. Theses types of reservoirs may be considered prime potential candidates for UBD operations. However, UBD is a complex process and many factors must be considered and evaluated for candidate reservoirs before any operation.

Underbalanced technology may be very successful in reducing or eliminating formation damage if properly executed, but a major portion of this study has centered on problems, which are often associated with UBD and the development of detailed protocol for proper design and execution of UBD program 1. Two main goals are evaluated this study, improving the productivity of fractured reservoir by using UBD and reducing formation damage during UBD.

Possible formation damage effects may occur even if an underbalanced pressure condition is maintained 100% of the time during drilling operation. Another, one of the major areas of sensitivity to formation damage during UBD operations is the loss of the underbalanced pressure condition. Hence, it becomes vital to evaluate the sensitivity of the formation to the effect of an overbalanced pulse situation.

The best way to evaluate damage potential is to test representative field fluids and core samples under simulated downhole conditions, as is possible with dynamic formation damage (DFD) test apparatus.

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