Sandstone acidizing has been carried out with mud acid which combines hydrochloric acid and hydrofluoric acid at various ratios. The application of mud acid in sandstone formations has presented quite a large number of difficulties like corrosion, precipitation of reaction products, matrix deconsolidation, decomposition of clays by HCl, and fast spending of the acids. There has been a recent trend to use chelating agents for stimulation in place of mud acid which are used in oil industry widely for iron control operations. In this study, two chelates, L-glutamic-N, N-diacetic acid (GLDA) and hydroxyethylethylene-diaminetriacetic acid (HEDTA) have been studied as an alternative to mud acid for acidizing. In order to analyze their performance in the application of acidizing, coreflood tests were performed on Berea and Bandera sandstone cores. Another disadvantage of mud acid has been the fast spending at clay mineral surfaces leading to depletion of acid strength, migration of fines, and formation of colloidal silica gel residue. Hence, compatibility of chelates with clay minerals was investigated through the static solubility tests. GLDA and HEDTA were analyzed for their permeability enhancement properties in Berea and Bandera cores. In the coreflood experiments conducted, it was found out that chelating agents can successfully stimulate sandstone formations. The final permeability of the Berea and Bandera cores were enhanced significantly. GLDA performed better than HEDTA in all applications. The substitution of seawater in place of deionized water for mixing purposes also led to an increased conductivity of the core implying GLDA is compatible with seawater. In the static solubility tests, chelates were mixed with HF acid at various concentrations. GLDA fluids kept more amounts of minerals in the solution when compared with HEDTA fluids. Sodium-based chelates when mixed with HF acid showed inhibited performance due to the formation of sodium fluorosilicates precipitates which are insoluble damage creating compounds. The application of ammonium-based chelate with HF acid was able to bring a large amount of aluminosilciates into the solution. The study recommends the use of ammonium-based GLDA in acidizing operations involving HF acid and sodium-based GLDA in the absence of the acid.

Sandstone acidizing is a complex operation because the acidizing fluid reacts with a variety of minerals present in the formation that results in a wide range of reaction products. The hydrofluoric acid (HF) reaction rate differs widely from mineral to mineral because of the variation in the reaction rate and the area of contact with the injected fluid. The series of reactions occurring in sandstone makes it all the more difficult to find the exact individual reaction rate constants. An improved model that provides better estimates of the outcome of a sandstone acidizing treatment is developed following a review of previous sandstone acidizing models. The model follows the lumped mineral methodology and is based mainly on the kinetic approach. The use of accurate reaction-rate laws allows the model to effectively predict the consumption of acidizing fluid during the stimulation treatment. The consideration of a proper equation for the silica gel filming factor accounts for the fact that some clay becomes inaccessible to the acid when silica gel precipitates on their surface. The proposed model is shown here to be valid in extrapolating laboratory coreflood data and predicting the effluent acid concentration at various flow rates. The damage during sandstone acidizing can be minimized when stimulation treatments are designed according to the percentage of carbonate in the formation, type and amount of clay in the formation and the reservoir bottomhole temperature. Most of the available software for design and evaluation of acidizing treatments do not consider the temperature and mineralogy effects extensively. We studied one such software and developed recommendations to improve the design and evaluation of sandstone acidizing treatments by taking into account the multifaceted effects of temperature and mineralogy in increasingly deep and hot sandstone environments. These recommendations will be of great use in the times to come as most of the wells will have to be drilled at greater depths in search for new reserves.

Mud acid, which is composed of HCl and HF, is commonly used to remove the formation damage in sandstone reservoirs. However, many problems are associated with HCl, especially at high temperatures. Formic-HF acids have served as an alternative of mud acid for a long period. Several factors may influence the outcome of an acidizing job in sandstone formations. In this research, effects of mineralogy, temperature, and HF concentration were studied. Various clay minerals (kaolinite, chlorite, and illite) were examined to react with formic-HF acid mixtures which contain different concentrations of HF. Coreflood experiments on sandstone cores featured by different mineralogy with dimensions of 1.5 in. x 6 in. were also conducted at a flow rate of 5 cm^3/min. Formic or acetic acids were used in preflush stage to remove the carbonates. A series of formic-HF acid mixtures with different ratios and concentrations were tested, and temperature varied from 77 to 350 degrees F. Inductively coupled plasma (ICP), scanning electron microscopy (SEM) and 19F nuclear magnetic resonance (19F NMR) were employed to follow the reaction kinetics and products. Besides, acetic-HF acid system, which is another important alternative of mud acid, was also investigated to compare with formic-HF acids. The species and amounts of reaction products of different clay minerals in organic-HF acids depend on mineral type, acid composition and ratio, and this is further confirmed by coreflood experiments, in which sandstone cores with different mineral compositions give quite different responses to the same acid mixture. As preflush, formic acid becomes more effective in removing carbonate minerals in sandstone cores at higher temperatures. In main flush stage, more concentrated HF can react with more clay minerals, but can also cause higher risk of CaF2 precipitate. Both formic-HF and acetic-HF acids are much milder than mud acid. When reacting with clay minerals, there is no big difference in the behaviors of 13 wt% acetic-HF acids and 9 wt% formic-HF acids, as long as the HF concentrations are the same.

Acidizing sandstone reservoirs is a complex process. If not fully studied, it could lead to formation damage. A combination of HCl/HF has been widely used to stimulate sandstone reservoirs. However, the success rate is low due to the complexity of the reactions involved in this process. These reactions result in potentially damaging precipitation and cause formation damage. The problem is more severe when dealing with Bandera sandstone formations that contain a high concentration of carbonate minerals and clay particles. The purpose of this study is to present and evaluate multi-stage acid injection into the Bandera sandstone cores to remove formation damage. In this study, coreflood experiments were conducted on Bandera sandstone cores (1.5 in. x 6 in.) at a flow rate of 4 cm^3/ min and temperature of 140°F. A mixture of formic acid and HF was used as an organic mud acid. Preflush of hydrochloric and formic acid was employed to remove carbonate minerals. Bandera sandstone cores contain a considerable amount of HCl sensitive clays. So another stage was employed to cover clay minerals and prevent HCl attack on the surface of clay particles. Different clay stabilizers as well as preflush pore volume were examined in this study. At the end, this multi-stage treatment design was tested on a Berea sandstone core to investigate the impact of mineralogy. During each experiment effluent samples were collected. Samples were analyzed using Inductively Coupled Plasma (ICP) and Scanning Electron Microscopy (SEM) to investigate reaction kinetics and chemistry of precipitation. Chemical analysis confirmed incompatibility of HCl with clays in Bandera cores at 140°F. Clay stabilizer CSA showed the ability to prevent HCl attack on the clay particle’s surface. As a result, a coreflood experiment conducted using CSA led to permeability improvement. The result of the coreflood experiment conducted using CSC indicated that this chemical is able to exchange cations with clay particles, however permeability decreased due to an insufficient injection of preflush. As in another experiment, increasing preflush pore volume using CSC resulted in permeability improvement. CSB completely failed to cover clay minerals and permeability decreased drastically at the end of the treatment.