Abstract

A performance analysis technique is presented which can be used to identify recompletion candidates in producing gas fields. Through geological description and a field case history, it is demonstrated that p/z versus cumulative production curves can be diagnostic tools in identifying inefficient completions or wellbore damage in a stratified reservoir system. It is also shown that these wells can be recompleted or redrilled to add significant incremental reserves which would have otherwise been abandoned.

Previous theoretical work presented in the literature demonstrated that layered gas reservoirs producing without crossflow display non-linear p/z versus cumulative production curves. This paper presents field data which support that observation and concludes that the absence of this behavior in a stratified reservoir environment may be indicative of a recompletion candidate due to wellbore damage in one or more of the layers. A field case history is presented involving wells producing from the Mesaverde Group in the San Juan Basin of New Mexico. Wells which were originally completed with nitroglycerin were selectively recompleted to yield significant incremental reserves. These wells were identified by their linear p/z behavior in an area where theory and historical performance indicate a non-linear p/z curve should be present. The results of the recompletion program are presented.

Introduction

The Mesaverde Group is the most important producing horizon in the San Juan Basin. This group accounts for approximately half of the estimated ultimate recovery of the basin. Almost all of this production can be attributed to the Blanco Mesaverde Field of New Mexico and the Ignacio Blanco Field of Colorado.

A common completion practice used on Mesaverde wells during the early to mid-1950's was to detonate a charge of nitroglycerin at the bottom of the well. Presumably, this placed the charge opposite the productive interval. The explosion fractured or rubbled the formation and increased both the initial flow capacity and ultimate recovery of the well. The standard procedure was to complete the entire Mesaverde Group open hole. In many cases, the result was a very inefficient completion due to the nature of the explosive technique and the length (500–1000 feet) of the productive interval. Only the most porous sands directly opposite the charge were effectively opened to flow. The remaining sands in the upper portion of the open hole were not stimulated and, consequently, did not contribute significantly to the production of the well.

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