Abstract

The Horizontal San Andres play in Yoakum, Cochran and adjacent counties has been advancing now for 5 years with over 150 horizontal producing wells. This pseudo-conventional play developed to take advantage of lower lease costs, shallower depths, single horizontal legs from each vertical pilot well, scaled back frac design, and slower production decline rates. A possible resource play? Operators are finding significant success, but not without well-to-well and regional issues. There seem to be as many theories on drilling and completing these wells as there are operators. Although many of the problems have been attacked and tamed by inter-operator cooperation, there is a firm belief that both the rocks and, especially, the fluids play a key role in the success of the play, and that these have been given short shrift.

One of the keys to understanding the viability in the play is to correctly evaluate the importance of facies, diagenesis (early and late-stage), and fluids (both in-situ and produced) in a well/area. Typically these factors are evaluated separately, if at all. The best outcomes going forward will result from considering these as a vertically variable unit. We suggest calling this unit a PROFILE.

This profile includes: depositional history/facies analysis; understanding late diagenesis/impact of Mother Nature’s Waterflood (MNW); and saturation values and composition of in-situ oil, gases, and water, as they all have an impact on location, drilling, landing depths and completion decisions. These profile components are often part of the decision to drill a San Andres horizontal well, but too often looked at in detail only if/when a project fails to meet desired production results.

In our DOE sponsored project (DOE 10123-17), we developed a “cookbook” in with a summary the “classic” observations of ROZs in vertical wells, and presented an ROZ-based, revised interpretation of the properties. The “weight” of these observations may be different for horizontal projects. Two of the most striking are gas composition and GOR. There have been a number of horizontal wells where, as a result of large volume sweep/diagenesis, methane is not the primary component of the produced gas. H2S, CO2, and C2 – C6 can be very important components and have a significant impact on both early and sustained oil production. The gas/oil ratio (GOR) also appears to be critical, as both low and high GOR values may be detrimental to a successful project. High quality mudlogs (e.g., using mass spectrometry) are of immense value/essential for evaluating the gaseous components providing the drive mechanism.

The rational for where to land the horizontal leg is also not “set in stone”. The ROZ in some areas may have a three tiered saturation profile which is associated with facies distribution, degree of lateral sweep from MNW, and related diagenetic overprint. Finally, flexing of deep seated structures may have resulted in the development of shallower fracture clusters that have either been sealed by anhydrite, or been enhanced and became pathways for high volume flushing and/or created pathways where the frac can penetrate out of the target interval into porous, open marine facies with low-yield (low GOR) oil, below the paleo oil/water contact. Again, mudlogs are often be critical in identifying these fracture clusters.