Acid fracturing is a stimulation process in which the dissolution of formation rock leaves unevenly etched fracture faces that creates lasting conductivity after fracture closure. The success of acid fracturing depends on the conductivity created and retained under overburden stress in addition to the length of conductive fracture. In order to have sufficient conductivity after fracture closure, the fracture face must be non-uniformly etched by the acid while the strength of the rock is still maintained at high levels to withstand the closure stress.
A systematic experimental study to investigate the effect of etching pattern and rock strength on the resulting fracture conductivity was conducted in a laboratory facility designed to perform acid fracture conductivity characterization. Detailed etched surface characterization and rock strength measurement were performed on acid etched fracture faces in order to correlate to resulting conductivity.
Resulting fracture conductivities clearly show the importance of characterizing acid etching patterns. In evaluating the measured fracture conductivities, we distinguished between those tests in which a prominent channel was created along the middle of the rock samples from those tests in which more uniformly distributed roughness features were created on the fracture faces. The channels developed in small scale laboratory tests are likely artifacts of the apparatus and cannot be expected to scale to field conditions.
For the roughly etched fracture faces, statistical parameters describing the surface roughness in addition to average fracture width were needed to accurately predict measured conductivities. Both etched pattern characterization and resulting conductivity measurements suggested the need to model conductivity of limestone and dolomite samples separately. New conductivity correlations were developed both for limestone and dolomite formations which matched the experimental results better than previous correlations.