Understanding Wormholes in Carbonates: Unprecedented Experimental Scale and 3D Visualization

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Abstract Matrix acidizing experiments combined with visualization techniques commonly are used to study the details of wormhole networks formed during matrix acidizing of carbonate reservoir rock. Previous experimental studies of wormhole growth 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 provided extensive information on linear 1D wormhole growth along with some basic insights into 2D radial growth mechanisms. However, larger-scale test systems must be considered if 3D wormhole characteristics are to be understood. Toward that end, a new method was developed that integrates acidizing experiments on carbonate rock samples up to 14 ft3 in volume, high-resolution nondestructive imaging and analysis, and computational modeling to extend the results of experiments to field applications. This article highlights the experimental and imaging components. Introduction Substantial hydrocarbon volumes have been and will continue to be produced from carbonate formations, which hold nearly half of the world's reserves. Because 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 longterm 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 on core plugs are conducted to determine the optimum conditions to generate wormholes (i.e., highly conductive flow channels that connect the nearwell region 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 Fig. 1. The figure shows an example of a ’wormhole efficiency curve?? developed for core plugs of quarried limestone. 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.

Electrokinetic Driven Low-Concentration Acid Improved Oil Recovery in Abu Dhabi Tight Carbonate Reservoirs

“…Matrix acidizing (with acid concentration of up to 15%) commonly enhances production by increasing the effective wellbore radius and causing stimulation as seen in earlier large scale studies [7]. Matrix acidizing experiments combined with visualization techniques are commonly used to study the details of wormhole networks formed during matrix acidizing of carbonates [8]. Several treatment parameters such as acid injection rate, acid concentration, and variation of rock permeability, reservoir temperature and acid volume injected are optimized to execute an efficient acidizing job in carbonate reservoirs [9][10].…”

Fundamental controls on fluid flow in carbonates: current workflows to emerging technologies

“…At the opposite end of the spectrum, we perceive further opportunities to design experiments in the subsurface or in large laboratory facilities to learn more about deformation and fluid-rock interaction behaviours over larger volumes. For example, McDuff et al (2010) demonstrated the benefits from experiments on wormhole formation via the non-destructive imaging of large (c. 0.5 m 3 ) blocks of carbonate rock.…”

Reaction Kinetics and Coreflooding Study of High-Temperature Carbonate Reservoir Stimulation Using GLDA in Seawater

“…The acid type affects the structure of wormholes generated during carbonate acidizing [7,12]. Slow reacting acids, such as chelating agents and other organic acids form wider, less branched wormholes, while highly reactive acids, such as HCl, form highly branched wormholes [13]. The relative magnitudes of acid transport and reaction rates define the controlling step of the dissolution reaction whether kinetically controlled or mass-transfer controlled [14].…”