Monitoring of Matrix Acidizing by Use of Resistivity Measurements
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This paper describes the testing of a novel concept based on resistivity measurements to monitor acid-stimulation operations. It is believed that the proposed concept for monitoring the wormholing process can be adopted in the field with the deployment of induction tools. The outcome of this novel monitoring concept is expected to provide an unprecedented level of understanding of the depth, number, and type of wormholes being created downhole.
Induction-logging tools can be viewed as an attractive option for characterizing wormhole morphology resulting from the acidizing process and can be used to assess acid-stimulation operations. The authors propose to extend the use of resistivity-logging tools for evaluating acid-stimulation jobs after well cleanup. This concept relies on the significant variations in the electrical resistivity of the different fluids and chemicals involved in the acidizing process and the increase in the effective porosity in the near-wellbore region resulting from the acid reactive dissolution. In this work, the authors conducted resistivity measurements while acidizing carbonate core samples. To do so, an electrically sensitive coreflooding setup was designed to conduct acidizing tests of carbonate core samples while measuring the change in the electrical resistivity at multiple points along the core and in real time. The paper shows the potential use of such measurements to monitor wormhole penetration and branching in real time. A yard test was conducted to verify the response of a real induction tool to simulated wormholing features.
Resistivity Measurements While Coreflooding
Experimental Setup and Procedure. A three-phase methodology was followed: rock-sample characterization, coreflooding and resistivity measurements, and characterization of the core samples after acidizing to inspect the wormhole structure and invert through the model to close the experimental loop.
Saturation of Cores and Porosity Measurement. The core samples were put under a vacuum for a few hours and weighed. Then, they were saturated with 50,000 ppm of sodium chloride (NaCl) at a pressure of 2,000 psi for a few hours. After saturation, the samples were weighed again; the difference in the weight of dry and saturated samples was simply converted to the pore volume of the rock.
Permeability Measurement. The permeability of the rock samples was measured by the constant-flow-rate method. This was performed by flowing deionized water across the cores in the coreflooding system while recording the pressure drop. The permeability was computed from the pressure drop and corresponding flow rate using Darcy’s law.
Coreflooding Experiments. The dual-coreflood system used for the acidizing experiments is a high-pressure, high-temperature core-flow test apparatus that comprises a fluid-delivery pump, four fluid accumulators, and two core holders, each capable of holding a 12‑in. core, placed inside an oven. The rubber sleeves inside the core holders have four equidistant pressure sensors (2.4 in. apart) connected to pressure transducers. These transducers can thus monitor the pressure difference across a particular section. Consequently, the propagation of a wormhole can be recorded as a function of pressure drop in a particular section. Furthermore, eight electrical-resistivity tappings (1.25 in. apart) were placed along a custom-molded rubber sleeve housing the core sample and were connected to a meter that measured inductance, capacitance, and resistivity. The procedure used for each of the core-flow tests is detailed in the complete paper.
Computed-Tomography (CT) Scan. Dry cores were scanned using X-ray CT with a slice thickness of 1 mm, thus generating approximately 300 slices of each core. The cross-sectional images of the rocks were obtained with an image-analysis and visualization software product and were used to visualize the wormhole morphology of the acidized core samples.
Results and Discussion
A series of coreflooding experiments was conducted to investigate wormholing in carbonates under different operating conditions.
Pressure Analysis and Post-Acidizing Core Characterization. The differential-pressure and resistance data were monitored while injecting acid through the core samples. A typical pressure response (normalized with respect to preacidized values) is obtained from the equally spaced transducers in the core sleeve as the acid is injected into the core samples. These measurements allow for the estimation of breakthrough for each 2.4-in. section of the core sample as the wormhole forms and propagates. The breakthrough volume and time were computed when the differential pressure at each transducer section dropped to zero. A pH meter was also used to corroborate the point of acid exit from the outlet, to confirm breakthrough.
After the coreflooding experiments were completed, the cores were soaked in fresh deionized water to be desalinated. Next, the core was dried at a temperature of 65°C in an oven for approximately 24 hours. The cores were then taken for the X-ray CT analysis. All cores were scanned under the same room conditions and with the same examination plan. For low injection rate (0.5 mL/min), the authors observed a thick wormhole propagating along the core. For intermediate injection rates (1 and 2 mL/min), a thinner wormhole was obtained. Furthermore, the authors observed some tortuosity in the fluid conductive path resulting from reactive dissolution, indicating the presence of heterogeneity in the porous medium. For higher injection rates (5 mL/min), highly branched wormholes were obtained, indicating more transverse flow of the acid and deviation from the main wormhole.
Resistivity Measurements. The use of resistivity to monitor the wormhole propagation and profile is a novel concept that relies on the significant variations (up to three orders of magnitude) in the electrical resistivity of the different fluids and chemicals involved in the acidizing process. The work flow used to obtain the dynamic resistivity measurements while conducting the coreflooding experiments is described here.
Brine was injected until stabilization was observed in the resistivity data. These present the baseline preacidized values. Then, acid injection proceeded until a complete breakthrough was achieved. Finally, brine was reinjected until stable resistivity readings were obtained. A sharp decrease in the resistance values was observed when the acid penetrates the zone under inspection. Resistivity at the first channel does not drop immediately after acid injection. In fact, wormholes do not form immediately when the acid comes into contact with the inlet face. During the wormhole-initiation phase, the acid penetrates into the matrix pores and enlarges them as the reactive dissolution takes place. The pores grow, and, after exceeding a critical pore size, they give rise to wormholes. As the wormholes propagate, the effective porosity of the core sample increases, more acid flows through it, and then the electrical resistance decreases. Interestingly, the decreasing trend observed during acid injection undergoes two different slopes. The first slope is associated with the wormhole propagation, and the second corresponds to the wormhole growth and transverse branching within the area of investigation. The initiation of the first slope is indicative of the level of acid penetration within the core. The second slope can be used to characterize the wormhole structure; a steep slope is indicative of the formation of transverse branches and wormhole-thickness growth as the acid flows through the respective core zone. The stabilization of resistance at higher values occurs after reaching the complete breakthrough and reinjecting brine.
The derivation of a numerical model to determine electrical resistivity while injecting acid and dissolving the rock sample is detailed in the complete paper.
Yard Test: Response of Induction Tool to Simulated Wormholing Features
A yard test using an array-induction-imager tool was carried out to demonstrate further the capability of resistivity measurements to monitor acidizing operations in the field.
Test Procedure and Results. Preparation of Wormholing Jigs. Jigs consisting of 1 m (length)×8 in. (diameter) polyvinyl chloride (PVC) pipes were fabricated to simulate different wormholing features, as shown in Fig. 1. Transverse pipes of varying diameter and length were attached to the main PVC pipes. These pipes were filled with brine to mimic electrically conductive wormholes. Highly absorbent spill pads were used to hold brine as needed to reproduce a face-dissolution acidizing regime (Fig. 1a). The same amount of brine was used for all cases (1.5 L).
Test on Air With Conductive Borehole. The first yard test was conducted on air while placing the array-induction-imager tool in a 6-in. pipe filled with fresh water to mimic a conductive borehole. This test was carried out in the calibration area, which is cleared of any metallic parts that may affect the measurements.
Resistivity data were retrieved with an integrated field-acquisition system and processed with a wellbore software platform. In the obtained resistivity logs for all simulated wormholing cases, the array-induction-imager-tool curves obtained for the baseline case show high resistivity values because of borehole correction introduced to data processing. The mud resistivity is 1.3 Ω·m. All curves corresponding to different depths of investigation overlap. For Cases 1 and 2, the array-induction-imager-tool logs show characteristic shapes as the wormholing jig passed the receiver/transmitter pairs in the array-induction-imager-tool array section. This indicates the sensitivity of the induction tool to shallow wormholes. However, no changes were observed in the array-induction-imager-tool curves for Cases 3 and 4. In a highly resistive medium (air), the induced electromagnetic field from the array-induction-imager-tool transmitter was attenuated even in the presence of conductive transverse pipes simulating wormholes for Cases 3 and 4. These pipes need to be connected continuously through a conductive material so that the array-induction-imager tool can detect the wormholes.
Monitoring of Matrix Acidizing by Use of Resistivity Measurements
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