A Study of Fracture Conductivity in Carbonate Rock Created by Acid and Proppant
Abstract
Today the majority of onshore United States oil production comes from unconventional reservoirs that have very low permeability and porosity, such as the Austin Chalk formation, and require stimulation methods to produce economically. A very common method is hydraulic fracturing, which can be split up into proppant supported fractures and acid etched fractures. The cost of hydraulic fracturing depends on the type of fluid used as well the type of proppant. The proppant holds the created fracture network open after pumping has ceased and the closure stress of the reservoir is applied to the fracture network, allowing reservoir fluid to flow to the wellbore. Acid etched fractures apply the same principles as proppant supported fractures except that acid etched fractures rely on high points left after acid has dissolved the fracture surface to hold the fracture open.
A 40/70 mesh sand provided by SM Energy was tested for this study. Acid fracture conductivity created using 15% hydrochloric acid was also tested and compared with the propped fracture conductivity. Samples were tested that were acid fractured with 15% hydrochloric acid at 10-minute and 20-minute residence times and then propped with proppant using a concentration of 0.2 lbm/ft2.
The ability of fluid to flow through the fracture network to the wellbore is called fracture conductivity and it can be used to describe how successful a hydraulic fracture treatment has been or predicted to be if modeled. Dimensionless fracture conductivity is the ratio of the fracture’s ability to transport fluid through the fracture over the reservoir’s ability to transport fluid to the fracture. It is used to evaluate and optimize hydraulic fracture design. American Petroleum Institute Recommended Practice 61 procedures guided the conductivity testing of all the samples. This is the recommended practice for evaluating short term conductivity of propped and acid etched samples and has been put in place by the American Petroleum Institute. Proppant is the solid used in hydraulic fracturing that props the fracture open after pumping of the hydraulic fluid has stopped and the reservoir closure stress is applied to the fracture. The same concepts apply to acid etched surfaces where incongruent high points and dissolved faces hold the fracture open.
As recommended by American Petroleum Institute Recommended Practice 61, a series of closure stresses are utilized ranging from 1000 psi – 6000 psi in 1000 psi intervals to test the fracture conductivity. To obtain results related to the strength of a proppant, a sieve analysis is conducted before and after fracture conductivity testing. The sieve analysis provides a grain size distribution and quantitative analysis of proppant crushing.
It was concluded from this study that limestone reacts favorably to being acid fractured at low acid residence times and yields conductivity values higher than those of a propped fracture. The results indicate that a short acid pre-flush followed by proppant maximizes the conductivity of the limestone while longer acid pre-flushes minimize conductivity as the dissolution weakens the rock’s ability to support the fracture. This study offers an insight into the conductivity of limestone formations and how acid affects the near-wellbore conductivity while presenting a detailed methodology for quantifying fracture conductivity in a laboratory setting.
Subject
Fracture conductivityCitation
Fojtasek, Alexander (2022). A Study of Fracture Conductivity in Carbonate Rock Created by Acid and Proppant. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /197091.