Using Horizontal Well Technology for Enhanced Recovery in Very Mature, Depletion Drive Gas Reservoirs - Pirkle #2 Well, A Case History, Carthage (Lower Pettit) Field, Panola County, Texas
Authors: McCoy A.W. , Davis F.A. , Elrod J.P. , Rhodes S.L. , Singh S.P. 
Published: December 2007

Abstract
Horizontal well technology has been successfully applied to exploit reservoirs involving thin beds, low permeability zones, naturally fractured reservoirs, high-cost areas, and zones of water coning. The Pirkle #2 well represents the first use of horizontal technology to enhance ultimate gas recovery in a very mature, low pressure zone in the Lower Pettit horizon at Carthage Field, Panola County, Texas.
The Pirkle #2 well was drilled to test the concept that a horizontal well could enhance ultimate recovery by lowering the final abandonment pressure in a very mature, depletion drive gas reservoir. However, numerous technical obstacles existed to the successful drilling and completion of an economic well in a 0.0308 psi/ft pressure gradient environment. This paper outlines the steps taken by OXY team members in planning and executing the project, as well as the results achieved from the Pirkle #2 well. Information gained from this project will help others to define appropriate screening criteria and provide guidance for planning/application of horizontal technology to other mature gas reservoirs worldwide.

Introduction
Carthage Field is located in Panola County, Texas as shown in figure 1. The field encompasses over 300,000 productive acres and produces from seven different formations, three of which, the Upper Pettit, Lower Pettit and Cotton Valley have produced 2.1, 3.8 and 2.0 TCF, respectively.
The productive intervals of the Lower Pettit member in Carthage generally consist of a series of porous and permeable, linear-trending, northwest-southeast oolitic beds. The Lower Pettit pay zones, first discovered in 1936, occur between the depths of 5,900 and 6,100 feet, and originally contained 4 TCF of gas in place. The Lower Pettit reservoirs at Carthage cover over 267,000 acres, an area approximately 48 miles long by 24 miles wide, and have been produced by over 470 wells. A map of the net porous limestone thickness and the productive area limit is shown in figure 2. As of January 1996, there were 199 wells producing with a total average daily production of 34 MMCFD from the Lower Pettit.
Many of the older Lower Pettit wells have been abandoned due to low production rates (less than 60 MCFD). These wells were generally located in thinner pay intervals within the field. Due to the drilling of wells to the deeper Cotton Valley sands within the last 20 years, new log information has been obtained over the Pettit zone which has significantly increased our understanding of this formation. In OXY's portion of the field, several recent replacement wells drilled in thicker pay sections have resulted in substantial improvement in well deliverabilities over the older wells. It was this discovery that lead to the idea of drilling a horizontal well to improve ultimate gas recovery.

Reservoir Description
The Lower Pettit pay is described as an ooid coated-bioclast grainstone having variable intergranular primary and intragranular secondary porosity. Variations in porosity are illustrated in a thin section photograph of the Lower Pettit as shown in figure 3. These dramatic variations in porosity and permeability are common throughout the Pettit pays at Carthage and they are important factors to consider in determination of well location.
A typical well log across the Pettit formation is shown in figure 4. The Lower Pettit member on OXY's acreage is divided into two main pay zones, whose in-house names are Frost "A" and Pirkle A-2. The Frost "A" is the major producing zone in the area around the Pirkle #2 well. General reservoir characteristics for the Lower Pettit are listed in Table 1.
In OXY's portion of the field, the Frost "A" pay zone is essentially a large depletion drive reservoir. Early in the field life a few gas wells along the southwestern edge watered out during the late 1950's and early 1960's. However, the aquifer ceased to encroach during the mid 1960's and this zone has basically behaved as a depletion drive since that time.
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