Rebirth of a 30 Year Old, Offshore Gulf of Mexico Field Using Horizontal Drilling in Ultra Thin Oil Reservoirs
Authors: Simms G.J. , Hedberg R.L. , Dwyer J.P. , Flecker M.J. 
Published: December 2007

This paper discusses the pre-drill evaluation and post-drill results of a four well horizontal drilling program in two bottom water drive reservoirs with 20 foot oil columns. The pre-drill evaluation consists of a multidiscipline reservoir interpretation that combines 3-D seismic, stratigraphy, oil correlation, reservoir model simulation, and performance. Factors critical to the successful execution of the drilling program were hole size, rig selection and drill-in fluids. Program results are derived from measured while drilling open hole logs, well performance, flowing pressure data, and production logging. The paper analyzes the complexities of production log interpretation in two phase horizontal flow.

Introduction

The West Delta Blocks 75/90 Field lies on the eastern flank of the West Delta 73 Complex. Located in the Offshore Gulf of Mexico, 85 miles south of New Orleans, it was discovered in 1963 (Fig. 1). The Field produces from 22 sands ranging in age from Upper Miocene to Upper Pliocene (17,900'-5200' subsea).

This paper concentrates on the Lower Pliocene F-40 sand, the objective of three of the four horizontal wells (B-11st, B-17st, and D-19st). At the time the decision was made to drill the horizontal wells, seven wells were producing from the sand at a combined rate of 450 BOPD + 6500 BWPD. The high average water cut of 93% was a result of severe water coning due to a thin oil column overlying a large, active aquifer with good vertical permeability.

The F-40 sand has produced 9 MMBO (1965 - 1995) from 27 completions. Depletion of the remaining reserves would take in excess of 10 years with rates of less that 200 BOPD during the final 5 years (Fig. 2). Clearly, this was uneconomic.

The decision to drill three horizontal wells into the reservoir was based on the expectation that a horizontal well would:recover additional reserves by increasing drainage area,increase rate by contacting more reservoir and achieving higher PI's, andreduce water cut by reducing drawdown and thereby the cone.

Reservoir Evaluation

The structure at the F-40 level is a series of extremely low relief (&lt; 1 degree dip) north dipping three way fault closures (Fig. 3). The sand is approximately 300 ft thick with an original column height of 60 ft. The current column height is estimated to be 25' at the structural highs. The reservoir produces by strong water drive, having experienced only 300 psi of drawdown over the reservoir's 30 year life. Current reservoir pressure is 3510 psi (1996). Porosity and water saturations average 30%. Horizontal permeability is approximately 800 md. Although there are indications of stratigraphic variability that will be discussed later in this paper, consistent water levels, pressure similarities and performance indicate the sand produces as one reservoir.

Eight redevelopment campaigns have left the structure with 43 penetrations in the F-40 sand. The pre-1995 drill evaluation integrated a new, proprietary 3-D seismic survey with the existing well control, oil correlation data, reservoir model simulation, and performance. In order to support the location of additional wells in an already densely developed area, the reservoir team had to accurately identify subtle highs, define the current oil column, understand the reservoir quality and continuity, and determine the extent of interference from existing producing wells.

P. 133