A Comprehensive Model of High-Rate Matrix-Acid Stimulation for Long Horizontal Wells in Carbonate Reservoirs: Part II—Wellbore/Reservoir Coupled-Flow Modeling and Field Application

Abstract

Summary Successful acid stimulation of long-horizontal-well intervals in carbonate reservoirs requires effective acid distribution along the entire reservoir length. Such treatments also require large volumes of acid and seawater/brine injection at sufficiently high injection rates to drive the acid wormholes deep into the reservoir. Under these flowing conditions, significantly large tubing friction loss is anticipated unless optimal friction reducer performance in the tubing is maintained throughout the pumping operation. Because prediction of wormhole penetration and corresponding skin factor depends on analysis of downhole-injection pressures at the reservoir face, it is crucial to properly account for these hydrostatic and friction changes prior to evaluation of wormhole length and skin factor. In this study, an integrated flow model has been developed to predict the wellbore-pressure profile and wormhole distribution by tracking the movement of the acid in the wellbore and the formation. The wellbore-flow model is based on steady-state, 1D, pressure-based nodal method. The segmented wellbore in the reservoir interval is then coupled with analytical transient reservoir-flow models. The wormhole propagation in the formation is calculated based on the modified Buijse-Glasbergen correlation and upscaling model developed in our earlier work. The resultant wormholing skin factor is calculated by simulating and updating the changing well injectivity along the entire injection interval at every timestep. The model developed in this work is applicable for both fully completed wells (i.e., radial flow) and selectively completed perforation-cluster wells (i.e., spherical flow) typically employed in carbonate reservoirs. Analysis of injection rates and pressures during acid treatment provides engineers with a way to determine the varying injectivity and tubing friction as stimulation proceeds. The model presented here can be used as a forward model for analyzing real-time treatment rate and pressure histories and can also be used to review past treatments to improve future treatment designs. Using actual field-stimulation data, we also discuss key elements to successful stimulation planning and the diagnosis of matrix-acid treatments to achieve effective wormhole coverage for horizontal completions in carbonate formations.

Study of Acid Fracturing Strategy with Integrated Modeling in Naturally Fractured Carbonate Reservoirs

“…Due to the limitation of simulation scale and computing power, the global wormhole model is the best choice for simulating the wormhole growth during acid fracturing. This study adopts a semi-empirical wormhole model presented by Furui (2012) [19].…”

“…This model predicts the propagation rate of the wormhole region instead of individual wormhole characteristics. The most famous ones include the volumetric model (Economides et al, 1993) [17], the Buijse and Glasbergen model (Buijse et al, 2005) [18], and the Furui model (Furui et al, 2012) [19]. Hill et al (1995) [20] presented a leak-off model with a wormhole effect by combining Carter's leak-off model and the volumetric model.…”

Numerical Studies and Analyses on the Acidizing Process in Vug Carbonate Rocks

“…Before, lots of studies have been conducted to study the carbonate matrix acidizing process. Both experimental (Bernadiner et al, 1992;Wang et al, 1993;Frick et al, 1994;Fredd and Fogler, 1999;Izgec et al, 2008;Furui et al, 2012a;Furui et al, 2012b;Dong, 2012;Aidagulov et al, 2018;Qiu et al, 2018) and numerical (Kanaka and Panga, 2003;Panga et al, 2005;Kalia and Balakotaiah, 2007;Kalia and Balakotaiah, 2009;Maheshwari et al, 2013;Maheshwari et al, 2016b;Liu et al, 2019) results show that an optimum acid injection rate widely exists, at which the minimum amount of acid solution is consumed to achieve a breakthrough (Jia et al, 2021). Meanwhile, different dissolution patterns (Hoefner and Fogler, 1989;Fredd and Fogler, 1999;Huang et al, 2000;Izgec et al, 2008;Aidagulov et al, 2018) can also be observed through visualization techniques at different acid injection velocities.…”