Matrix acidizing is a well stimulation technique used to remove formation damage in the near wellbore region. But it comes with an associated set of challenges such as corrosion of the tubulars and iron precipitation in the formation. To counter these challenges, different chemicals, or additives, are added to the acid solution such as corrosion inhibitors and iron control agents. These additives may change the relative permeability of the spent acid, and formation wettability, and may either hinder or improve spent acid clean-up. Such effects of additives on the spent acid clean-up have not been documented. The aim of this research effort was to document the aforementioned change in the spent acid concentration (by using one additive at a time) before and after gas flowback. This was achieved by acidizing cores and creating wormholes halfway through them, then CT scanning them to observe the spent acid region. Later on, gas was flown through the core opposite to the direction of acid injection for 2 hours, and another CT scan was taken. The difference between the two CT scans was documented. Using a different additive each time, a series of such CT scans was obtained to develop an idea about whether the said additive was beneficial or detrimental to spent acid clean-up. It was found that the corrosion inhibitor FA-CI performed the best in terms of spent acid recovery after gas flowback for both Indiana Limestone and Texas Cream Chalk cores. Moreover, the corrosion inhibitor MI-CI was the worst for Indiana Limestone and the non-emulsifying agent M-NEA the worst for Texas Cream Chalk for spent acid recovery after gas flowback.

Diversion methods are routinely used in both matrix acidizing and fracturing stimulation treatments. In this study, we focus on one of the classical mechanical diversion methods, ball sealers. Ball sealer diversion is used in cased and perforated wells to divert stimulation fluids by temporarily blocking perforation holes in the casing with rubber-coated balls. This diversion method can be very effective, but there is no general methodology to design ball sealer diversion, or to evaluate its effectiveness from the treating rate and pressure record. Experimental data from an extensive series of full-scale flow experiments conducted by BP were analyzed. One field treatment was analyzed and compared, and a similar trend in seating efficiency was observed. Then, we developed an empirical correlation on the basis of BP experimental data. The correlation enables us to estimate how many ball sealers seat on the perforations. By incorporating this correlation into an acid placement model, we can estimate wormhole penetrations along the wellbore with time. We developed a computer program and studied one hypothetical case to show the usefulness of the ball sealer diversion model.

Core flooding is a commonly used experimental procedure in the petroleum industry. It involves pressurizing a reservoir rock and flowing fluid through it in the laboratory. The cylindrical rock, called a core, can be cut from the reservoir during a separate core drilling operation or a formation outcrop. A core flooding apparatus suitable for matrix acidizing was designed and assembled. Matrix acidizing is a stimulation technique in which hydrochloric acid (HCl) is injected down the wellbore below formation fracture pressure to dissolve carbonate (CaCO3) rock creating high permeability streaks called wormholes. The main components of the apparatus include a continuous flow syringe pump, three core holders, a hydraulic hand pump, two accumulators, a back pressure regulator, and two pressure transducers connected through a series of tubing and valves. Due to the corrosive nature of the acid, the apparatus features Hastelloy which is a corrosion resistant metal alloy. Another substantial feature of the apparatus is the ability to apply 3000psi back pressure. This is the pressure necessary to keep CO2, a product of the CaCO3 and HCl reaction, in solution at elevated temperatures. To perform experiments at temperature, the core holder is wrapped with heating tape and surrounded by insulation. Tubing is wrapped around a heating band with insulation to heat the fluid before it enters the core. A LabVIEW graphical programming code was written to control heaters as well as record temperature and pressure drop across the core. Other considerations for the design include minimizing footprint, operational ease by the user, vertical placement of the accumulators and core holders to minimize gravity effects, and air release valves. Core floods can be performed at varying injection rates, temperatures and pressures up to 5000psi and 250 degF. The apparatus can handle small core plugs, 1’’ diameter X 1’’ length, up to 4’’ X 20’’ cores. The equipment description includes the purpose, relevant features, and connections to the system for each component. Finally documented is the procedure to run a core flooding test to determine permeability and inject acid complete with an analysis of pressure response data.

The main purpose of acid/water jetting treatments currently applied in the field is to clean up the filter cake formed during the drilling process and perhaps further stimulate the wellbore by creating wormholes if acid jetting is used in carbonate formation. This purpose can be achieved for the reason that the filter cake on the borehole can be mechanically broken by the high speed jetting action, and additionally, if acid is used, some materials in the filter cake can be dissolved, which can facilitate the mechanical breaking action. The knowledge of jetting effectiveness under various conditions is crucial for the purpose of optimizing the treatment design. In order to investigate quantitatively the effectiveness of acid/water jetting for filter cake cleanup and wellbore productivity enhancement, laboratory experiments were carried out under conditions similar to those in the field. Filter cake was deposited on the face of a 4 inch diameter core and then water or 15% HCl were used for jetting treatment. The original permeability, the permeability right after the drill-in fluid damage, and the permeability after the jetting treatment were measured and compared. The effect of overbalance pressure during the jetting treatment was investigated. CT scan was carried out for those cores that may have wormholes after the acid jetting treatment. An analysis of the mechanism for filter cake removal and wormhole creating during acid jetting treatment was proposed. It is discovered that acid jetting can effectively remove the filter cake by penetrating and lifting it from beneath, and efficient wormhole creation can only happen when the overbalance pressure during the acid jetting treatment is above a certain value. Based on this study, several suggestions for field applications were made.

In matrix acidizing, the goal is to dissolve minerals in the rock to increase well productivity. This is accomplished by injecting an application-specific solution of acid into the formation at a pressure between the pore pressure and fracture pressure. A hydrochloric acid solution is used in carbonate reservoirs, which actually dissolves the calcite rock matrix in the form of conductive channels called wormholes. These wormholes propagate from the wellbore out into the reservoir, bypassing the damaged zone. In matrix acidizing of carbonates, there are four parameters that affect performance: the concentration of calcite present, injection rate of the acid, reaction type, and heterogeneity. Of these parameters, this paper will focus on how rock heterogeneity affects performance. To do this, a coreflood and acidizing apparatus was used to acidize heterogeneous limestone core samples. Rock characterizations and volumetric measurements were considered with the results from these experiments, which made it possible to correlate and quantify the results with rock and volume parameters. It was found that the core samples with more and larger heterogeneities generally required less acid (measured in pore volumes) to achieve breakthrough, that is, a wormhole created axially from one end of the core to the other. This value for pore volumes to breakthrough was one to two orders of magnitude less than more homogeneous samples. The general procedure and best practices for acidizing the core samples is also detailed in this thesis. This procedure was followed for preparation, coreflooding, and acidizing for all core samples.

The Middle East has 62% of the world’s proved conventional oil reserves; more than 70% of these reserves are in carbonate reservoirs. It also has 40% of the world’s proved conventional gas reserves; 90% of these reserves are hold in carbonate reservoirs. Recently papers published from industry discussed the techniques, planning, and optimization of acid stimulation for Qatar carbonate. To the best of author’s knowledge, no study has focused on the acid reaction to Qatar carbonates. The lack of understanding of Qatar carbonate especially Middle East carbonates and the abundance of Middle East carbonate reservoirs is the main motivation behind this study. This work is an experimental study to understand the acid response to Qatar rocks in rocks with two types: homogenous carbonate and heterogeneous carbonate. A large portion of this research is to further investigate the impact of centimeter scale heterogeneity on the acid stimulation using Qatar rocks. Qatar carbonates have multi-scale heterogeneities which may cause the impact of the injected acids to differ from homogenous case. Recent published field data indicate a much smaller number of pore volume to breakthrough compared with experimental measurement with homogeneous carbonate and heterogeneity is believed to be one of the contributors of causing the low field measurements. In this case, acid linear core-flood experiments were conducted with carbonate core samples of different petrophysical properties to study the impact of both separated and connected vugs and channels on pore volume to breakthrough. Computerized tomography was used in characterization of the heterogeneities. One experiment simulated the response of acid to heterogeneous carbonate in downhole condition with drill-in fluid damage. Homogeneous rock was cored from a well in Qatar. The optimal injection rate was pursued through acid core flood experiments for acid stimulation design and for further reference. It is been discovered that the optimum injection rate for heterogeneous carbonate exists. For the similar acid flux, the corresponding PVBT for buggy limestone correlates inversely with the fraction of total porosity comprised by vugs. For vuggy carbonates with connected vugs and channels, whether or not formation damage exists, the acid tends to create new pore space nearby to the existing vugs and channels. Different strategies need to be made regarding acid stimulation design with homogeneous carbonate, heterogeneous carbonate with separated vugs and channels and heterogeneous carbonate with connected vugs and channels.

The goal of this work was to evaluate matrix acidizing treatments of vertical and horizontal wells in carbonate reservoirs. Twenty field cases for acidizing treatments were analyzed by evaluating the skin factor evolution from on-site rate/pressure data during the treatment. A skin monitoring method based on the concept of inverse injectivity (Hill and Zhu, 1996) was used to calculate the skin factor evolution. Viscous diversion techniques were analyzed by using the viscous diversion skin model that accounts for viscosity contrast between the reservoir fluid and the injected fluid. The estimated skin evolution during the treatment was validated using the post-treatment well performance. From the post-treatment analysis, it was observed that emulsified acid was not an efficient viscous diverter because only 27% of the wells treated with emulsified acid showed evidence of viscous diversion. Therefore, other viscous diversion techniques are needed to ensure uniform acid coverage. In addition, treatments that involved diversion techniques such as foam, associative-polymers, and viscoelastic surfactants were also evaluated. Thus, the post-treatment evaluation was used to improve and optimize the acid treatment designs. This study was beneficial to diagnose if excess acid volumes were used, or effective diversion was achieved during the acid treatment.

Design of matrix acidizing treatments of carbonate formation is still a challenge although extensive research has been done on it. It is necessary to estimate acid distribution along the wellbore. This estimation is very important especially for the case where the reservoir properties vary along the wellbore. This work provides development and application of an apparent skin factor model which accounts for both damage and mobility difference between acid and gas. Combining this model with a conventional acid placement model, we develop an acid placement model for vertically extensive, multilayer gas reservoirs. A computer program is developed implementing the acid placement model. The program is used to simulate hypothetical examples of acid placement for vertically extensive, multilayer gas reservoirs. This model will improve matrix acidizing for gas reservoirs and enable realtime monitoring of acid stimulation more accurately.

In live acid solutions at concentrations of HCl namely 15-20 wt% HCl, elastic modulus remained quite low as compared to 10-12 wt% HCl concentrations. At 10 wt% HCl concentration, elastic modulus was 3.4 Pa observed whereas at 20 wt% HCl concentration, elastic modulus was 0.03 Pa. 0.5- 1.0 wt% concentrations of NaCl and CaCl2 showed negligible effect on the elastic modulus while 3-10 wt % concentrations, substantially reduced the elastic modulus. As little as 0.5 wt% Fe (III) concentration reduced elastic modulus quite significantly. In live acids, increase in temperature resulted in viscous modulus dominating the elastic modulus. Corrosion inhibitor reduced values of elastic modulus significantly, at 10 wt% HCl concentration elastic modulus dropped from 5.1 Pa to 3.4 Pa. Preparation of acid solution with sea water showed negligible effect at higher concentrations of HCl (> 10 wt% HCl) whereas at lower concentrations of HCl the elastic modulus fell sharply. For spent acid solutions, the elastic modulus at room temperature was quite low. Increase in temperature resulted in the increase in elastic modulus up to 130 F after which it decreased. At 190 - 205F and 18.8 rad/s, elastic modulus for 12 wt% HCl concentrations was 0.4 Pa whereas at 130 F, it was 2.25 Pa. At high temperatures (>130 F), the maximum elastic modulus shifted to higher concentrations of HCl namely 20 wt% HCl concentration. At 160 F, elastic modulus of 20 wt% HCl concentration at 18.8 rad/s was observed to be 2.6 Pa, whereas for 12 wt% HCl concentrations, it was 1.27 Pa. Throughout the HCl concentration and temperature range tested, viscous modulus dominated the elastic modulus for spent acid solutions. The effects of organic acids namely, formic and acetic acid, on the elastic modulus of viscoelastic surfactants have also been investigated.

Matrix acidizing is a well stimulation procedure where acid is injected down the wellbore or coil tubing and into the reservoir near the wellbore region. Wellbore damage is a common issue in the oil field. The primary goal of matrix acidizing in carbonate reservoirs is to bypass wellbore damage by creating highly conductive channels that go several feet into the formation, known as wormholes. The goal of laboratory experiments is to find an optimum injection rate to create dominant wormholes and provide this information to the field. To conduct various experiments, core flooding setups are created. The setup consists of a core holder, accumulator, overburden pump, injection pump, accumulator, pressure sensors, and a back pressure regulator. Results from matrix acidizing core flooding in laboratory conditions provide an understand for wormhole growth, acid diversion, injection rates, and adds a variety of liquid chemicals for testing at reservoir pressures and temperatures. The first objective was to design, assemble, and test a matrix acidizing parallel core flooding apparatus. The apparatus was rated for 5,000 psi and 250 ºF. Combinations of the various mechanical components were chosen appropriately to meet the requirements. Electrical wiring and data acquisition hardware was assembled. LabVIEW software code was written for controlling temperature and recording data. The second objective was to create a documented method for conducting experiments.