No abstract

Matrix acidizing is a common stimulation method for wells in carbonate reservoirs. Typically, 15% HCl in water, and some additives for specific purposes, is injected into carbonate rocks to create wormholes to improve hydrocarbon flow. These wormholes are formed by acid dissolution of the carbonate rock. The path by which the wormholes are formed is normally of lowest resistance. The path of rock dissolution is governed by permeability heterogeneity and normally follows the path of highest permeability. There are other aspects that could influence the path of the wormhole, that is mineralogy heterogeneity. When an acid is exposed to an insoluble mineral, it is expected that it will change its path and could deviate, even from the path of highest permeability. Effects of this process on the stimulation treatment are expected to vary from one rock type to another. The objective of this study was to compare the path of the wormhole in rocks with homogenous mineralogy with that of rocks with heterogeneous mineralogy. In both cases, the permeability distribution in the rock was analyzed and compared to the path of the wormhole. The ultimate goal was to define whether the presence of acid insoluble minerals affects the path of the wormhole. Micro-CT scanning at various resolutions was utilized to visualize the pore space connectivity. Thin section analysis of rock samples was used, in combination with micro-CT images, to identify mineralogy at various locations. This was conducted with the support of scanning electron microscope (SEM) analysis. Thin section analysis was conducted for the same rock samples that were scanned and the results were compared so that CT-images can be calibrated and used to identify mineralogy of other similar rock samples. The results were beneficial to the research team as more information about the rock properties from CT-scan images were obtained.

In this paper, we present our interpretation for some of the unexpected experimental results that we obtained during 3D spontaneous imbibition experiments. We treated carbonate rocks with flourochemical polymers where we altered the wettability towards reduced liquid-wetting. The expected result is a reduced imbibition rate as a result of reduced capillary forces. Although the early imbibition rate decreased, we observed an increase in rate at late imbibition time resulting in a higher liquid saturation in the altered rock. We used numerical modeling to interpret the result and show that this observation actually represents the target wettability state we should be seeking in some applications.

A well-designed acid stimulation treatment is a rapid well intervention operation that can lead to long-trim economical benefits. In carbonates, the main challenge is to better design acid stimulation treatments in order to meet the two main characteristics of carbonate reservoirs which are high water production and a wide range of heterogeneities. Acid diversion is one of the key factors for a proper distribution of acid which determines the success of these treatments, especially in heterogeneous carbonate formations. In this study, we aim at producing novel solutions in order to reduce water production and enhance zonal coverage through effective diversion. The ultimate goal is to validate a single pumping sequence that combines water mitigation, diversion and acid injection. In order to achieve this, we rely on an integrated R&D approach combining single core and dual core experimental setups. The advantage of the dual core setup is to investigate different permeability contrasts, different saturations configurations and to experimentally validate wormholing in heterogeneous carbonates. In fact, in carbonate effective diversion happens when acid is flowing in the low permeability area at sufficient velocity. An easy way to validate this is through parallel core flooding. We performed a series of high pressure and temperature single and dual core flooding experiments over a range of different carbonate samples having different permeability and porosities. The injection took place at 90 °C and 250 bars. We assessed the efficiency of polymer adsorption by measuring permeability during, before and after polymer injection. Computed Tomography scans were used in order to validate our conclusions. We showed that, adsorption of polymer in carbonates is possible but needs careful design. We experimentally investigated acid diversion in high permeability contrasts (matrix-fissures) through core flooding. This is another novelty in this study. Finally, the combined pumping sequences were experimentally assessed. We validated diversion in carbonates and identified the optimum injection rates required for such applications. An optimum configuration of permeability contrasts and saturations exists for a combined pumping sequence. The experimental results and observations are translated into a series of guidelines and procedures which are directly applicable to acid stimulation field operations. The areas of future required research in applied fluid-rock interactions will be highlighted.