Historically, vertical wells were used to correlate formation tops and determine the lateral continuity of the reservoir. With the advancements in horizontal drilling and logging, the industry is able to gather an immense amount of information about the rock as we drill farther away from the vertical section. Numerous industry publications indicate that approximately 40% of the perforation clusters in hydraulic fracturing do not contribute to production. Many factors play a role in such production behavior, but the most important factor is the breakdown of perforations and propagation of the hydraulic fractures through them. Several methods, such as limited entry design and placing perforations in similar type rock, have been applied to mitigate this problem; the information needed for these methods is obtained from logging the laterals or using drilling data to determine rock properties. Diagnostic tools such as production logs, permanent downhole fiber optics, radioactive tracers, and chemical tracers have been deployed to understand the varying production profiles seen across the unconventional reservoirs.
This study focuses on three wells with lateral measurements to obtain petrophysical and geomechanical rock properties (one well in the Wolfcamp B and two wells in the Wolfcamp A). The wells also had pseudo rock properties calculated using surface drilling data. In most instances, the perforation clusters in each stage were placed in good reservoir and completion quality rock with the aim to minimize the stress differential between clusters. Different perforation schemes were tested in each of the three wells - number of clusters and spacing, limited entry, and geometric design. The wellbore geosteering profile, whether in or out of zone, was also considered in relation to the subsurface structure.
Lateral measurements in all wells showed the changing lithology and rock types across the lateral. The Wolfcamp B had a production log that indicated twice as many clusters contributing in the section of engineered perforations compared to the section where the perforations were placed using the gamma ray log. Time-lapse chemical tracers in other wells indicated changing production profiles. For example, early in the life of a Wolfcamp A well, the stages with clusters picked based on logs showed the highest production contribution compared to the geometric stages, but, later, the trend started to shift in favor of the geometric clusters. The geometric stages were in an area of the wellbore where the carbonate content was highest.
Comparisons of various data sets to production performance, such as the one included in this study, will provide some insight into the heterogeneous nature of the Wolfcamp shale and the impact of varying perforation techniques on production contribution from individual clusters.