Stratton Field, Texas Gulf Coast: A Successful Cased-Hole Re-Evaluation of an Old Field to Determine Remaining Reserves and to Increase Production Level
Authors: Libson T.E. , Vacca H.L. , Meehan D.N. 
Published: December 2007

Recompletion decisions at Stratton field historically have been complicated by the presence of many thin sand lenses of widely varying pay quality. The advent of a cased-hole formation evaluation tool-the dual detector pulsed neutron log (DDPN)-has simplified matters greatly by revealing current, detailed information about reservoir rock properties and fluid saturations. By using 4 years of recompletion data, DDPN log characteristics and log derived parameters have been identified and used successfully to distinguish between gas-, oil-, and water-bearing zones, to predict when fracture stimulations will be required to produce at commercial gas rates, to tify depleted gas zones, and to rank potential recompletion zones according to their respective absolute open-flow potentials (OFP's). potentials (OFP's).

Introduction

Stratton-Agua Dulce field, discovered in the late 1930's, is a gulf coast field located about 45 miles [72 km] west of Corpus Christi, TX (see Fig. 1). Production is from the Frio and upper Vicksburg formations (Oligocene shaley sands), with the typical wellbore penetrating more than 25 reservoirs scattered throughout a 3,000-ft [914-m] interval (see Fig. 2 for a type log). The reservoirs are relatively thin sand lenses, ranging from 5 to 20 ft [1.5 to 6.1 m] gross interval. The areal extent of these resvoirs is limited by both the many down-to-the-coast faults in the area and reservoir pinchouts attributed to sand depositional environments. Thus recompletion attempts have been complicated not only by the sheer number of reservoirs, but by the complex geology of the area. It is quite common that recompletion attempts reveal a sand that is wet, depleted, or tight but productive in an ediate offset well. Likewise, sands known to be wet or depleted in some wells often are found to be commercial in offset recompletion attempts. With the increase in hydrocarbon value since 1978, recompletion econonmics have improved vastly and created a need for a tool to help identify the best possible zone remaining in the wellbore. The original suite of openhole logs (for all the older wells this consisted of simply an electric log) was not found to be useful since production from a given reservoir in offset wells could raise a water table into the prospective zone or pressure deplete it. Migration of gas between zones behind pipe also has been found to occur. The tool with the highest degree of success in recompletion attempts is the DDPN. Specifically designed to be run in a cased hole, it consists of a high-energy neutron source and two detectors in a logging sonde. The DDPN had been accepted in areas of the Texas gulf coast with highly saline water before 1978; however, the lower Stratton water salinities (15,000 ppm total Cl ) had delayed use of the log in Stratton. After some experimentation it was determined that indeed the DDPN could be used in Stratton since, although not fully understood, the DDPN routinely recorded capture cross-section values (E) in the range of 24 to 30 c.u. for Stratton's wet, shaly Frio sandstones. Equivalent to a much larger indicated water salinity (50, OW to 80, OW ppm total Cl ), this allowed the required resolution to discriminate between hydrocarbonand water-bearing sands. Additionally, the newer generation of DDPN equipment offered both porosity indicators and gas detection. The DDPN, therefore, was selected for cased-hole recompletion analysis and subsequently assisted in both selection of recompletion zones and updating the condition of producing reservoirs. Theoretical calculations based on DDPN tool responses are used to calculate rock porosities and fluid saturations of behind-pipe zones. Schlumberger's experiences with recompletion results since 1978 have determined certain log characteristics and log-derived parameters useful as guidelines to predict the parameters useful as guidelines to predict the commercial potential of a candidate recompletion zone in Stratton field. They used a (effective porosity times water saturation) product of 10% or less as indicative of a commercial hydrocarbon zone. This paper refines the service company pay cutoff by establishing correlations between actual recompletion attempt results at Stratton and the corresponding DDPN curve readings. Over the last 4 years, we ran 27 DDPN logs in Stratton field (see Fig. 3) and attempted a total of 76 recompletions across the logged intervals.

JPT

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