Profiling Potential Petroleum Production
Oil often has to be coaxed out of the ground, but various conditions hinder pumping and decrease production. One problem might be that the oil cannot flow freely through the rock. One way to stimulate a well is to inject acid, which cuts channels through the rock, allowing oil to move more freely and, thereby, boosting output. To be useful, however, any increase in conductivity spurred by acid etching of the rock must be significant and lasting.
Not all rocks react the same way to acid. Here, three types of rock — limestone, dolomite and chalk — are mapped using a custom profilometer. All three samples started out smooth and relatively featureless, a result of sawing. After being subjected to acid, they are different. Visual results are on the left, and color-coded surface profiles from the laser-based profilometer are on the right. Measuring and analyzing such differences could help predict the effect of acid treatment on the flow of oil through rock formations. Courtesy of A. Daniel Hill, Texas A&M University.
Unfortunately, the industry has had only empirical correlations between improved conductivity and acid treatments. These rules of thumb have been based on limited data, noted A. Daniel Hill, a professor of petroleum engineering, partly because of a lack of good ways to characterize the surface of acid-etched rock fractures. Therefore, he and his colleagues at Texas A&M University in College Station set out to relate surface texture of rock to the conductivity created by acid treatment, with the goal of correlating these characteristics to acid-injection conditions in the field.
To do so, they developed a laser-scanning profilometer that can accurately gauge the vertical dimensions of an acid-treated rock surface to ~25 μm. They took as their starting point commercial laser profilometers, most of which are designed for the submicron requirements of the semiconductor industry. Although the engineers had no need for that degree of accuracy, they did like the concept enough to develop their own solution.
“We chose this technology because there are commercial instruments for measuring surface profiles based on the same principle. We designed and built the profilometer, including software, in a couple of months,” Hill said.
The custom profilometer used a laser displacement sensor from Acuity Laser Measurement (now part of Schmitt Industries Inc.) of Portland, Ore. This sensor fires a laser at a target and collects the reflected light via a lens that focuses the light onto a CCD. From the location of the spot on the imaging array, software calculates the distance to the target. In the case of the custom profilometer, that translates into an accuracy of a few tens of microns.
After sawing core samples of various types of rocks into uniform shapes and sizes for their setup, the scientists acid-etched each sample’s face and used the profilometer to map it completely. They then evaluated the conductivity by measuring the pressure of nitrogen that they flowed through pairs of the samples.
The research, which Hill reported at the 2006 Society of Petroleum Engineers Technical Conference, is ongoing, but it has already revealed some surprises — specifically, the different textures created by different types of acids on particular rocks. The group plans to detail some of these results in future reports.
Although improvements could be made to the profilometer, he noted that the surface measurements are helping the researchers visualize and interpret the effects of acid treatment — information that could lead to boosts in oil production. “I am very hopeful that completely new approaches to predicting acid-fracture conductivity will result,” he said.
Contact: A. Daniel Hill, Texas A&M University, Department of Petroleum Engineering, College Station; +1 (979) 845-2278; e-mail: firstname.lastname@example.org.
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