Eagle Ford Huff ’n’ Puff Single-Well Pilot Uses Large Y-Grade Injection Volumes

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 224381, “Eagle Ford Huff ’n’ Puff (EOR) Single-Well Pilot With Large Y-Grade Injection Volumes,” by Kevin DeLapp and Jonathan Baron, SPE, BlackBrush Oil & Gas, and Quoc P. Nguyen, SPE, The University of Texas at Austin. The paper has not been peer reviewed. _ The Eagle Ford at Chittim Ranch in the Maverick Basin has long been recognized for its tremendous amount of original oil in place (OOIP). To unlock the true potential of this reservoir, an exploration and development company began a 3-year study to design and implement an enhanced-oil-recovery (EOR) project using huff ’n’ puff (HnP). However, unlike many EOR/HnP projects that use field gas, the company targeted another source of injectant: Y-grade. This is the largest undertaking of a Y-grade injection project known to the authors. Introduction The geologic setting within Chittim is best summarized as a Cretaceous thrust structure (Chittim Anticline) that overlies a narrow Jurassic rift basin (Fig. 1). However, it is within a complex depositional and tectonic history that the lower Eagle Ford interval experiences expansion, thermal maturation, and uplift to an accessible present-day depth of approximately 2,500–6,000 ft below the surface. Oil gravity ranges from 28 to 40 °API. The quality of the Eagle Ford on the Chittim can be characterized by the amount of OOIP, which is approximately 68 million bbl per section. Because of the shallow depth, low absolute pressure, and low API gravity at Chittim, the historical primary recoveries achieved thus far have been underwhelming despite the high OOIP. Evolution of Development Plan Y-grade was chosen as the preferred injectant because of its many advantages over field gas, including first-contact miscibility, favorable relative permeability, solvent benefits, and operational and capital efficiencies. The operational advantages of Y-grade include the ability to pump it as a liquid vs. expensive high-pressure compressors; diminished containment risks, especially compared with the high minimum miscibility pressure required for field gas; and enhanced safety and less-hazardous equipment. A single, isolated well with no direct offsets (except for a sister well at the heel) was selected for the pilot. The pilot well had been producing for approximately 7 years before the start of HnP and was producing approximately 18 B/D. This began an over-3-year study to design and implement the company’s first HnP pilot. The production data, fluid properties, and fracture modeling in the pilot area were used to develop a mechanistic and numerical model of the stimulated reservoir volume (SRV) for compositional simulation of the HnP process. HnP Pilot Design The first consideration in the design was the amount of Y-grade needed to reach fill-up. The history match of historical production was used to estimate the size of the SRV, which would inform the approximate amount of Y-grade required. As a secondary check, the expected fill-up was presumed to be nearly equal to that of the cumulative oil produced since first production. For reference, before HnP, the pilot well had a cumulative oil production of approximately 112,000 bbl.