Acid Fracture and Fracture Conductivity Study of Field Rock Samples

Acid fracturing is a well stimulation strategy designed to increase the productivity of a producing well. The parameters of acid fracturing and the effects of acid interaction on specific rock samples can be studied experimentally. Acid injection data and fracture conductivity measurements obtained in the research presented in this thesis yielded results that qualified and quantified the impact of a specific acid system on rock samples of varying acid solubility.

Six rock samples from a carbonate reservoir were labeled A through F to protect proprietary information included in this research. A 2% potassium chloride solution was used for the acid system and fracture conductivity measurements to prevent clay swelling. Injection temperature, contact time, and injection rate were designed to simulate field treatment conditions. The effects of a chelating agent on fracture conductivity were also studied.

Before and after images of the rock samples indicated that the effect of 15% hydrochloric acid on the samples was limited but correlated with the rock acid solubility. Samples E and F had a greater value of acid solubility and showed noticeable surface etching. Samples A, B, and C had lower values of acid solubility and did not show signs of surface etching. Sample D was of moderate acid solubility and showed minimal signs of surface etching. Fracture conductivity did not correlate directly with acid solubility, but likely was a function of inherent matrix permeability based on leak-off measurements and fracture conductivity measurements. Finally, the fracture conductivity of Sample D increased after exposure to a chelating agent.

Commonly, acid fracture experimental studies are carried out with outcrop rock samples. The samples have more homogenous properties and without hydrocarbon content. In this study, cores from downhole formation were used. The original condition was preserved as much as possible to simulate real field situations. However, using field rock samples does present challenges not generally associated with outcrop rock samples.

Based on the information gathered from the work presented in this thesis, conclusions were drawn concerning the effectiveness of a 15% hydrochloric acid treatment in this formation and the challenges of using field rock samples.