(74o) Interaction of Oil and Gas Shale with Drilling Muds in Fracturing Fluids

The boom in drilling and completion in shale formations has been partially responsible for the large increase in oil production in the United States. To a large extent, development of shale resources has been driven by the use of high-rate fracturing treatments. These treatments typically use friction reducers to lower surface pressure requirements, and thus job costs. Probably the most common polymers used are polyacrylamides. While this family of polymers provides excellent friction reduction, they often interact with shales in a manner that potentially causes formation damage problems. The polymer – shale interactions are well known and have been used for years to stabilize shales during drilling. Since shales typically have low permeability, it is difficult to ascertain the magnitude of formation damage by polyacrylamide based fracturing fluids.

In this work, methods to quantify the propensity of polymers to stick to shale, and thus quantify fluid-rock interactions for systems where returned permeability measurements are difficult or impossible to make have been developed. This paper is focused on the interaction of shales from different formations (Gorgas, Woodford, Chattanooga, and Albany) and bentonite with varying concentrations of anionic and cationic polyacrylamides. The shale samples are ground using a micromill and the powdered shale sample is sieved using a sieve shaker and each shale sample with < 75μm particles is used for the experiments.

The concentration of each shale is varied for a fixed concentration of anionic polyacrylamide. The increase in viscosity with increase in shale concentration is recorded and the shale concentration at which hydrodynamic interactions are overcome and when the shale starts to interact strongly with the anionic polyacrylamide is determined. The experiments are repeated with cationic polyacrylamide.

Scanning Electron Microscopy (SEM) is done for shale samples. Before doing SEM the shale samples are dried at 60 º C for 24 hours using a vacuum oven. Gold was sputtered onto the samples and images of each sample was taken at different magnifications. In order to determine the elements that are present in each shale EDS analysis of these samples was carried out using Evex Nanoanalysis software on scanning electron microscope. The shale samples were deposited on carbon coated aluminum sheet. The instrument reported the presence of elements in oxide and pure state. The elements were then mapped onto the SEM images.

To have a better understanding of the interaction between the shale and polyacrylamide ESEM analysis was done for shale in polyacrylamide sample. 0.2 wt. % of each shale in 0.14 wt. % of anionic polyacrylamide was used for the test. The main advantage of ESEM is to analyze liquid samples at low pressure conditions. ESEM allows us to study the sensitivity for shales to polyacrylamides. ESEM is an important tool to validate the fluid/rock interaction in real time.

From the experiments, it is concluded that bentonite interacts strongly with anionic and cationic polyacrylamides whereas shale interactions are minimal with cationic polyacrylamides higher molecular weight anionic polyacrylamides interact more strongly. Chattanooga shale interacts most strongly and Woodford shale has least interaction with anionic polyacrylamide. This result is validated by SEM and ESEM. SEM images show that Chattanooga shale is more porous in nature as a result of which it has a larger surface area to interact with anionic polyacrylamides and vice versa for Woodford shale. Addition of salts such as NaCl/KCl prevents the anionic polyacrylamide interacting strongly with the shale.  The minimum concentration of anionic polyacrylamides at which shales start to interact is determined for each shale sample. Bentonite forms strong gels with polyacrylamides while all the shale samples form considerably weaker gels with polyacrylamides.

In this work the rheological properties of shale-polyacrylamide slurries are used to measure the strength of the polymer-shale interaction. The rheological properties of shale slurries are compared with the rheological properties of the bentonite slurries. The surface charge and elemental composition of each shale is determined. The critical overlap concentration and the critical entanglement concentration of high molecular weight anionic polyacrylamide are determined which will be used as a base for further research.