Abstract

Highly laminated tight gas sand sequences remain prolific targets worldwide and have often been bypassed using standard petrophysical analysis and simple porosity cut-off technique. The problem becomes more acute in marginal tight gas reservoirs. The high cost of hydraulic fracturing increases the need for an effective and useable petrophysical model for an accurate productivity indication of the target interval. The pressure to avoid non-economical completions continues to leave hydrocarbons bypassed. Using recent advances in logging technology and production optimization modeling, the thinly laminated gas bearing permeable sands can be discerned from clay dispersed in silt and sand. A true net height can now be obtained. Through production optimization modeling, it is possible to assess the economic viability of completing and stimulating highly laminated interval. In this paper, we will show a case study from a South Texas tight gas sand field. Several wells were evaluated using micro-resistivity imaging. From this an enhanced high-resolution petrophysical analysis was created. This image-enhanced evaluation of reservoir properties was combined with production modeling. The production performance was simulated for each interval and used for recommendations on completion strategies. Additional pay intervals, normally bypassed, were perforated and hydraulically stimulated. We compared production data from offset wells that used standard petrophysical analysis to the results of the newer wells with the high-resolution analysis. The results indicated that actual field production increased and paid out the increased fracture stimulation cost. Production logs acquired across entire intervals confirmed that portions of the field produced from horizons that were previously bypassed. This process is useful for any highly laminated tight gas sand sequence and is of widespread applicability. Previously bypassed intervals can now be assessed and completed effectively and economically. This process will further add to the reserve base of tight gas.

Introduction

Identification of low resistivity from highly laminated sand/shale sequences has been an exploitive success in recent years. Many proposals are put forth showing offset production that seem to defy standard log analysis and cut-offs. They show water production from saturation calculation that may be in the 70 or 80 percentile ranges. On top of this, these laminated sequences may be found in what are considered marginal reservoir rock even by today's standard and higher anticipated natural gas prices. These are the pay sands that are still only partially exploited and bypassed on an ongoing basis.

In tight, clay laminated gas sand basins, one technique that the completion engineer often uses to assess whether a zone should be attempted for stimulation is to perforate and flow the well. This luxury does not exist for the tight marginal gas sands discussed here. The risk associated with stimulating a non-flowing zone results in bypassed pay of unknown potential. Presented here is a proven technique for quantifying pay and sizing hydraulic fracture design options. The net height of laminated sand evaluated is compared with post-frac well production data to determine if the additional perforated intervals and larger stimulation treatments result in additional value. The methodology to compare production forecasts using a laminated sand analysis and stimulation designs is a useful tool for exploiting bypassed tight gas.

Geographical and Geological Background

The geographical setting for this case study is LaSalle County in South Texas. The area is known for the Olmos sand which has been exploited for tight gas primarily since the 1970's but initially since the 1920's.1 Another sand existing in parts of the Olmos play is known as the Escondido sand. There are a few nearby stand alone Escondido fields for comparison; Mesquite, Clark and Encinal. Kundert and Smink described recompletion stimulation techniques for the Mesquite field at a total depth of 5600 ft.2 The wells were natural producers in 1976 and stimulation efforts in 1977 led to substantially higher initial production rate and an estimated two-fold increase in cumulative production.

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