An Evaluation of CO2 Huff 'n' Puff Tests in Texas
Authors: Haskin Helen K. , Alston Robert B. 
Published: December 2007

Summary
Field experience in 28 Texas CO2 huff ‘n’ puff projects is presented and discussed. In the absence of mechanical problems, CO2 huff presented and discussed. In the absence of mechanical problems, CO2 huff ‘n’ puff can recover oil from 23- to 30 degrees API [0.92- to 0.88-g/cm3] Texas gulf coast Miocene reservoirs. Shorter soak times (10 to 17 days) recovered as much oil, or more, as longer soak times for 2- to 3-cp [2- to 3-mPas] crudes. Injection of larger volumes of CO2 (8 MMscf [230 × 10(3) m3] instead of 4 MMscf [110 × 10(3) m3]) resulted in greater incremental oil recovery of a 33-cp [33-mPas] crude. Oil-cut response can guide in the selection of wells to receive multiple cycles of CO2.
Two simple predictive methods are presented for estimating incremental oil recovery from CO2 huff ‘n’ puff. One is from the literature and the other was developed for the Texas reservoirs, where oil swelling and viscosity reduction are important oil recovery mechanisms. Although predictions from both methods show modest agreement with field production, predictions from both methods show modest agreement with field production, the method developed specifically for the Texas cases has the advantage of being based only on fluid properties, which are easy to measure or to estimate accurately.

Introduction
Although most of today's CO2 EOR projects involve large-scale continuous injection of CO2 solvent, there is increasing interest in cyclic CO2 injection into single wells. Typically, the rapid injection of CO2 (or CO2/hydrocarbon blends) is followed by a shutin period. The well is then returned to production and the response monitored. In reservoirs with poor interwell communication, this single-well approach may afford the only means of recovering tertiary oil by a CO2 process. In reservoirs where interwell communication is not a problem, CO2 huff ‘n’ puff offers a fast, inexpensive alternative to traditional EOR methods.
The engineer faced with designing a CO2 huff ‘n’ puff project can find only a limited amount of prior experience in the literature. Laboratory studies in two different 14 degrees API [0.97-g/cm3] crudes have indicated that CO2 huff ‘n’ puff will recover oil. The addition of nitrogen or methane as contaminants to the CO2 is not desirable because it reduces oil recovery. The optimum number of cycles is reported to be two or three, judging from field experience in Arkansas and a simulation study of California crudes. Bottomwater reservoirs and vertical fractures through which CO2 migration can occur should be avoided.
Numerical simulation has been used to design and to interpret projects. This is appropriate but is cost-effective only for large projects. This is appropriate but is cost-effective only for large projects. Low-cost predictive methods based on minimal input data projects. Low-cost predictive methods based on minimal input data are more suitable for the small field with a brief project life.
To increase our data base of field experience, this paper presents the results of 28 Texas wells stimulated by CO2 huff ‘n’ puff. The effects of reservoir parameters and design variables are discussed. Two simple predictive methods, one developed by Patton et al. and the other developed by us, will be evaluated.