The primary goal of this study was to couple reservoir characterization of the Abo Formation with hydraulic fracture analysis and subsequently, to infer infill drilling potential. Two detailed case studies were investigated to reevaluate original hydraulic fracture treatment designs, to compare and evaluate fracture parameters, and to determine if the fracture treatments were providing sufficient reservoir stimulation.
Approximately 80 wells were studied in the southern part of the Pecos Slope Abo Field. Decline curve analysis was performed on all wells by a modified set of Fetkovich type curves. Analysis of this work show linear to near-linear flow in most cases with permeability values less than 0.1 md and variations in reservoir properties as is typically observed in low permeability reservoirs. These variations resulted in difficulty in evaluating infill drilling potential.
Evaluation of fracture stimulation treatments was accomplished by matching recorded surface treating pressure with a fracture propagation model. Unusually high initiation pressures were observed in the case study wells; initial stress state, vertical fracture growth, fracture toughness, perforation restrictions, and the development of multiple fractures in a single, bounded layer were evaluated as possible causes for the high initiation pressures. Results from this work showed the multiple fractures model providing the best match for the two wells. Fracture length varied from approximately 1150 ft to 750 ft with a single fracture model and was reduced by approximately 10 to 30% for each additional fracture added. The fracture data was input into a fractured well performance model, which was used to match the production rate and therefore verify the process.
The integration of decline curve analysis and fracture analysis provided better descriptions of reservoir properties and more accurate designs of fracture propagation models. This combined approach also improved evaluation of infill drilling potential by evaluating reservoir properties and stimulation.
Background
The Pecos Slope Abo Field is located in southeastern New Mexico in north central Chaves County, directly north and east of the City of Roswell (see Figure 1). The producing area of the Abo formation is located on the northwestern part of the northwest shelf of the Delaware Basin. The structure is a gently eastward-dipping homocline, which occurs over the entire producing area2. The depositional facies of the Abo were influenced by the positive and negative features of the rise of the Pedernal highlands to the west, which had a marked effect on sedimentation during the late Pennsylvanian and early Permian3. The upper lithologic unit is approximately 600 ft thick and is composed of interbedded mudstones and lenticular sandstones; deposited in a fluvial deltaic system. The gas-bearing sandstones are very fine-grained, arkosic and hematitic4.
The Pecos Slope Abo Field was originally discovered in June of 1977 but development did not escalate until the early 1980's. In May of 1981 the Abo Formation was designated, as a tight gas formation. As a result of the tight gas designation, the Abo Formation became the leading target for natural gas in New Mexico in late 1981. The well spacing was restricted in the past to 320-acres but currently is 160-acres. Well spacing is reduced in order to increase the drainage area and improve recovery. Currently over 800 wells have been drilled in the Pecos Slope Field.
