Impact of Remedial Acid Practices on Proppant Embedment and Fracture Face Permeability
Authors: Karazincir Oya , Li Yan , Williams Wade , Zaki Karim , Rijken Peggy , Rickards Allan 
Published: October 2020

Acid stimulation is a common remedial practice for formations that produce below the target production rates. Acid stimulation can be performed during the early-life of a well, soon after the well is put on production, or more typically, following a production period once production rates have started to decline. Often, acid treatments are repeated multiple times during the producing life of a well, as restimulation becomes necessary. Sandstone acid systems can contain organic acids and/or hydrochloric acid, HCl, to target damage caused by carbonate solids, inorganic scaling, residual gel from fracturing operations or fluid loss control events, while hydrofluoric acid, HF, is used for siliceous damage/fines removal, mainly from and around the proppant pack in a hydraulically fractured formation. In a fractured formation, the acid sequence will first penetrate the least resistant zones, i.e. the fracture, and leak into the fracture face. For a propped fracture, proppant embedment at the fracture formation face naturally occurs under closure stress and increases with depletion.

A test program was designed to evaluate the effect of remedial acidizing practices on proppant embedment, frac face damage and reduction in porosity and permeability of the formation in these zones. A test method that can directly measure fracture-face permeability in parallel to fracture conductivity has been used to measure acid exposed fracture face permeability and proppant pack conductivity under depletion conditions. Initial tests were conducted in intermediate permeability, medium strength sandstone rock where baseline proppant conductivity and fracture face permeability were established with synthetic brine flow. Next, a remedial acid system was injected into the fracture and was allowed to leak-off into the fracture face. Following a shut-in period, brine was flowed back across the core platen representing the formation and the permeability of the core was tracked as a function of time under constant stress. Proppant pack permeability was also measured with brine flow along the fracture. The acidizing, shut-in and flow-back sequence was repeated three times to measure the impact of multiple acidizing treatments on proppant embedment, frac face damage and permeability loss. A similar test was conducted with five sequences of acid injection for comparison. Post-test analysis was conducted to study frac face damage, depth of proppant embedment and damage zone, rock strength and presence of fines in pore space. Test results were compared to those conducted with unacidized core.

The impact of repetitive acidizing practices on stimulation at the fracture face and on proppant embedment was investigated using the modified Frac Face Damage (FFD) test set-up, a test system that was introduced in earlier studies. At constant closure stress, FFD tests with acid injection can show permeability increase, followed by a decline in permeability. Current results suggest rate of permeability decline might accelerate after multiple acid treatments. Post-test analysis of the fracture face core has been conducted to evaluate the cause of the permeability reduction. Scanning Electron Microscopy (SEM) analysis show dissolved core material and fines being carried into the fracture face with one test. Micro CT-scan analysis is used to track porosity reduction at the fracture face. UCS and triaxial measurements are conducted to compare the change in the rock strength to that of the unacidized core. Thin section analysis of the fracture face core shows wide-spread grain crushing that penetrate deeper into the fracture face compared to unacidized core material tested with the same equipment.