Understanding Wormholes in Carbonates: Unprecedented Experimental Scale and 3-D Visualization
- Darren McDuff (ExxonMobil Corporation) | Chris E. Shuchart (ExxonMobil Upstream Research) | Shalawn Jackson (ExxonMobil Upstream Research) | Dieter Postl | James Seay Brown (ExxonMobil Corporation)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 19-22 September, Florence, Italy
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 2 Well Completion, 5.3.2 Multiphase Flow, 2.2.2 Perforating, 5.8.7 Carbonate Reservoir, 1.6.9 Coring, Fishing, 4.1.2 Separation and Treating, 1.8 Formation Damage, 3.2.4 Acidising, 4.3.4 Scale, 5.5.2 Core Analysis
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The combination of matrix acidizing experiments with visualization techniques is commonly used to elucidate the details of wormhole networks formed during matrix acidizing of carbonate reservoir rock. Previous experimental studies of wormhole growth have focused mainly on small linear core plugs, with only a limited number of radial flow studies published in the literature. Results from these conventional experiments have provided extensive information on linear wormhole growth in one dimension (1-D) along with some basic insights into radial growth mechanisms in 2-D. However, larger-scale test systems must be considered if 3-D wormhole characteristics are to be understood. Toward that end, a new methodology has been developed which integrates (1) acidizing experiments on carbonate rock samples up to 14 ft3 in volume, (2) high-resolution nondestructive imaging and analysis, and (3) computational modeling to extend the results of experiments to field applications. This article highlights the experimental and imaging components of the methodology.
Substantial hydrocarbon volumes have been and will continue to be produced from carbonate formations which hold nearly half of the world's reserves. Since these formations are highly soluble in acid, matrix acid stimulation provides a cost-effective means to enhance well productivity. Effective acid stimulation can be critical to achieving the desired long-term production rates from targeted reservoir layers.
Interaction of acids with carbonate rock has been studied extensively using quarried, outcrop, and formation core samples. Linear flow tests using core plugs are conducted to determine the optimum conditions to generate wormholes, i.e., highly conductive flow channels that connect the near-well area to the completion. At a given temperature, the ability of a particular acid to generate wormholes is largely dependent on the acid injection rate or "acid flux??, as illustrated in Figure 1. Shown is an example of a "wormhole efficiency curve?? developed for core plugs of quarried dolomite rock.1 As the curve indicates, there is a certain optimal acid flux for which wormholes will most efficiently propagate along the main axis of the core plug. Below the optimal flux, dissolution is mostly confined to the rock face nearest to the acid injection point; above the optimal flux, dissolution occurs more uniformly throughout the entire core plug rather than forming dominant individual wormhole channels. Computed tomography (CT) scan images of acidized core plugs show changes in wormhole shape along the acid efficiency curve.
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