--> --> A "Cookbook" Approach to Evaluating Residual Oil Zones completed as Horizontal Depressurizing (DUROZ) wells in the San Andres Formation

AAPG Southwest Section Annual Convention

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A "Cookbook" Approach to Evaluating Residual Oil Zones completed as Horizontal Depressurizing (DUROZ) wells in the San Andres Formation


In 2011 the authors presented a “Cookbook” for outlining the steps utilized to evaluate the potential for successfully CO2 flooding Residual Oil Zone (ROZ’s) in the San Andres with vertical wells. Since that time, a new unconventional San Andres Play has been developed utilizing horizontals wells and depressuring the upper ROZ (DUROZ). Although many of the concepts developed for evaluating CO2 EOR in vertical wells apply in the DUROZ play, there are some critical differences to approaching the two distinctly different plays. Both Brownfields and Greenfields ROZ’s are now being successfully exploited via CO2 EOR. The Tall Cotton Field in Gaines County has proven the concept of using vertical wells to flood a 300’ San Andres section of a Greenfield ROZ. Parallel with this vertical ROZ CO2 EOR development has been the development of the DUROZ play. This play, centered in Yoakum County, but with >350 wells spread across 6 counties in Texas and New Mexico is producing >35,000 BOPD, and has produced >25 million BO since 2011. The wells are typically frac’d in mile to 1.5 mile long laterals. The major differences from the vertical wells is in the landing depth, producing interval, frac designs, and production methods. Not all wells have been equally successful. The geometry of completions and drilling methods vary more significantly than in the vertical ROZ wells, however rock and fluid factors play a significant role in the success or relative failure of individual wells. Having reviewed the importance of the DUROZ components to the vertical CO2 EOR cookbook, is became apparent that there are critical differences in the evolving practices in the DUROZ: in drilling, mud logging, pilot hole requirements, core analysis, and completion. The same critical factors in controlling how/where to best complete a vertical CO2 well: depositional environments, diagenesis, sweep and saturations also apply to DUROZ horizontals, however, the weight of each varies. Technology and innovation has, once again, redefined the world of prospect opportunities. A comparison of the “classic” interpretation of “Goat Pasture” with the new understanding of Greenfield ROZ CO2 EOR, and DUROZ Horizontal development, and the science behind these new resources is presented.