Using Pressure Transient Analysis To Improve Well Performance and Optimize Field Development in Compartmentalized Shelf Margin Deltaic Reservoirs
Authors: Badgett K.L. , Crawford Gary E. , Mills W.H. , Mitchell S.P. , Vinson G.S. 
Published: December 2007

BP Exploration's Gulf of Mexico group developed procedures to conduct effective well tests on conventional production wells and employed them during the development drilling phase of the Mississippi Canyon 109 (MC109) field. Bottomhole pressure data were recorded during the initial few weeks of production. Typically, a 48 hour pressure buildup survey (surface shut-in) was obtained near the end of data acquisition. Data from these tests were analyzed for completion efficiency, reservoir flow capacity, reservoir heterogeneities, and drainage area.

Initially wells were gravel packed for sand control, until buildup interpretations indicated skins greater than 20. Frac packing technology was then employed, and an immediate improvement was observed with skins dropping into the teens. Over a period of time frac packs were optimized using the test derived skins as a metric.

Analysis of pressure data also played an important role in identifying reservoir compartmentalization. The two major reservoir horizons at MC109 are interpreted as shelf margin deltas. However, each of these has distinctly different compartmentalization issues. The continuous character of the G Sand made it easier to define the depositional system and investigate reservoir compartmentalization issues using a combination of well log, 3D seismic, static pressure trends, and fluid information. In the more distal deltaic reservoirs of the J Sand however, complications with seismic amplitudes and a less reliable tie between wireline and seismic data required the use of pressure transient analysis to efficiently exploit the reservoir.

Introduction

A well's production performance is a function of many variables: formation permeability, completion efficiency, reservoir heterogeneities, fluid properties, reservoir pressure, and drawdown. Well testing provides a means of quantifying these variables allowing the cause(s) of non-ideal well performance to be identified. Acquisition and timely analysis of bottomhole pressure data at the MC109 field facilitated improvements in completion efficiency, and helped identify and quantify reservoir discontinuities. Integration of this information with the field geologic model provided support for drilling at least two additional wells thereby improving field development.

Test Procedure

Since development drilling was conducted simultaneously with early production operations on the MC109 development, acquisition of well surveillance data introduced an additional complexity to an already intense operation. Operating under a "simultaneous operations" rule allows only two of the following operations to be conducted simultaneously: drilling, production, and well work (wireline operations). To minimize the impact of pressure transient testing on drilling and production, the procedure described below, was developed which added only a few hours to the well completion process and allowed gauges to be retrieved when operationally feasible.

The basic Gulf of Mexico completion is a gravel or frac pack. A typical well completion is shown in Fig. 1. Twenty-one of the thirty MC109 wells are deviated less than 65 degrees. In these wells, conventional wireline operations were possible. As part of the completion process, slick line was used to set test plugs, verify the tubing gauge, etc. As the final wireline operation, two electronic pressure gauges were run and "soft set" on a 1/4 inch mild steel pin welded across the shoe of the tailpipe below the sump packer. This arrangement allowed the gauges to be located out of the flow stream and to be retrieved without jarring.

These memory gauges were programmed to record pressures at ten minute intervals while the wells were jetted in and cleaned up. Several days after landing the gauges, the sample rates were increased to one reading per minute in one gauge and one reading every two minutes in the other.

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