The San Andres Play: Observations and Challenges in Horizontal Wells on the Central Basin Platform, Permian Basin
Authors: Alimahomed Farhan , Haddad Elia , Velez Edgar , Foster Randy , Downing Terrell , Seth Cody , Melzer Steve , Downing Will 
Published: January 2018

The San Andres is one of the most prolific conventional carbonate plays in the Permian Basin. It primarily occurs in the Central Basin Platform, but some fields are spread throughout the Northwest Shelf. The variation of the log profiles across the platform indicates a staged history and, challenging geological setting, which can have an impact on the lateral variability, landing zones and the completion techniques. Horizontal wells being a relatively new way to exploit this play, there are several challenges associated with making it economic. These challenges were faced in a program involving three horizontal wells on the Central Basin Platform. High tier petrophysical and sonic logs in the pilot, sonic and image logs in the lateral, and real-time microseismic data, were analyzed in the program.

Integrating data from various disciplines such as geology, petrophysics, geomechanics, completion engineering and reservoir engineering plays a significant role in identifying trends and key drivers of production. In the San Andres three-well program, high-tier petrophysical and sonic logging data were collected in the vertical pilot well. A fracture injection test (FIT) was performed to calibrate the rock properties. A 3D geomodel was built around the area of interest using well tops from offset vertical wells, and was refined to a localized structure around individual wellbores using dips from lateral image logs. Fracture simulations were performed to determine the optimum job size to cover the pay zone. Image logs in the lateral were interpreted for fractures and bedding planes, and to understand the changes in rock facies along the length of the lateral. Open hole sonic measurements in the lateral were used to place perforations in similar type of rock based on good reservoir quality and completion quality. Laboratory tests were performed on oil samples to determine the oil properties. Cuttings were analyzed to determine their solubility with acid. Two horizontal wells were monitored using real-time downhole microseismic. Post job analysis was performed to tie all the observations together.

Analysis of the injection test indicated slightly lower than normal reservoir pressure. Pilot-hole logs indicated a variable zone with mobile oil (pay) which was overlain on the top by anhydrite stringers and beds; higher water saturations were observed below the zone. The three horizontal wells in this program were all landed at various depths from the mobile oil interval to understand the impact on production. Step down tests were performed and analyzed on several stages to quantify near-wellbore friction pressures. Microseismic data showed planar features in stages that had fewer fractures identified on the image logs. High treating pressures were observed on alternate stages indicating some degree of stress shadow. Image logs in the laterals showed features such as anhydrite nodules and distinct layering of the rock, which can have a significant impact on the hydraulic fracture growth and also on production. The analysis of the fracture treatment and microseismic data yielded important information, and the program included the adoption of appropriate technologies and formulation of workflows for effective analysis.

The San Andres wells have been cost effective to drill and complete throughout the oil price downturn, but there are still many questions to be answered to make it an extremely successful play. The results and observations from this three-well program provide insights that will assist in planning and designing future projects.