Since the most common use of matrix acidizing is the removal of formation damage, it is important to understand the nature of the damage that exists so that an appropriate treatment can be designed. Well testing and well test analysis generate a skin factor and well completion efficiency. This is insufficient alone for formation damage diagnosis. Well performance analysis has provided a beneficial tool to identify the location and thickness of damage at flow points in the near wellbore area. Models of flow into perforations and gravel-packed tunnels provide a way to relate the location and severity of damage to the completion procedure that preceded it.

Treatment evaluation leads to problem identification and to continuously improved treatments. The prime source of information on which to build an evaluation are the acid treatment report and the pressure and rate data during injection and falloff. Proper execution, quality control, and record keeping are prerequisites to the task of accurate evaluation. Evaluation of unsatisfactory treatments is essential to recommending changes in chemicals and/or treating techniques and procedures that will provide the best treatment for acidizing wells in the future. The most important measure of the treatment is the productivity of the well after treatment.

ABSTRACT

This paper examines the influence of traces of oxygen on corrosion and hydrogen charging of steel in an H₂S containing environment. It is well known that H₂S promotes hydrogen entry into steels, that may result in many types of steel failures such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC), and Stress-Oriented Hydrogen Induced Cracking (SOHIC). Since it is a huge concern for oil and gas industries, standard test methods have been developed and published as NACE technical methods (e.g. NACE TM0284 and NACE TM0177). Though it is recognized that oxygen pollution should be avoided during H₂S cracking tests, there is still a lack of experimental data to illustrate the potential impacts of a small oxygen pollution.

The aim of the present study is to check if oxygen traces can modify the mechanisms of corrosion and hydrogen charging of steel in H₂S containing medium. Experiments consisted of hydrogen permeation measurements through a thin pure iron membrane. They were performed at free potential circuit in order to ensure more realistic environmental conditions. The corrosion rate was also evaluated and test solutions analyzed.

INTRODUCTION

Materials used in oil and gas industries can be exposed to sour environments containing hydrogen sulfide (H₂S), which is corrosive and known to promote hydrogen entry into steels. This may lead to several types of steel failures such as Hydrogen Induced Cracking (HIC), Sulfide Stress Cracking (SSC), and Stress-Oriented Hydrogen Induced Cracking (SOHIC).

Corrosion and hydrogen embrittlement of steels in H₂S containing environments has been studied for several decades. Standard test methods have been developed for the selection and the qualification of steels for use in H₂S containing environments, such as NACE TM 0177 and TM 0284.^1,2 These standards strongly recommend to avoid oxygen infiltration in test environments. For instance, it is stated that ‘obtaining and maintaining an environment with minimum dissolved O₂ contamination is considered very important’. It is also mentioned that O₂ contamination may induce an increase of the corrosion rate and reduce hydrogen evolution and hydrogen entry into the steel. However, it is also recognized that ‘systematic studies of the parameters affecting these phenomena have not been reported in the literature’.

Abstract Wellbores drilled through low-pressure formations encountered offshore or in depleted formations, require use of light-weight cement slurries (less than 13 pounds per gallon (ppg)). These densities in cements can be achieved through foaming, increasing the water content, or using silica-based microspheres. Water-extended cements have a threshold down to weights of approximately 13 ppg and to achieve densities lower than this require the use of foaming and/or silica based microspheres. Each of these methods has limitations that can severely impact hydraulic properties of cement. The foamed cements have the potential to become unstable at high pressures, while silica-based microspheres have chemical instability in the high alkalinity environment of wellbore. This chemical instability of silica-based microspheres used in cements, creates a hydrophilic gel that is expansive and creates fractures within cement matrix as it expands. This is more formally referred to as alkali-silica reactivity (ASR). Prevention of ASR involves the application of additives to the cement that act as a sink for the alkalinity during hydration for long-term prevention of the ASR. Lithium nitrate is one of these prevention methods that is theorized to allow for other beneficial reactions. This study investigates the effects of a highly alkaline cement pore-water on the chemical stability of microspheres. Microstructural characterization involves identification of reaction products in alkali- reacted glass beads within 28 days hydrated wellbore cement at wellbore temperatures, as well as the impact of lithium nitrate as a prevention method. The scanning electron microscopy of polished and fractured surfaces reveal two different reaction processes, with the ASR clearly absent in the slurry containing lithium nitrate. The micro-mechanical properties of these changes were also tested using microindentation tool. Lastly, porosity values were tested using helium gas Porosimetry. Lithium nitrate shows an effect on mechanical properties but not on porosity values as compared to cements solely containing microspheres.

Abstract

The purpose of the paper is the presentation of the geomechanical approach to the development and the improvement of methods for increasing well productivity and reservoir recovery. Control stress-strain state in the vicinity of a well to enhance the permeability of the well drained zone increases significantly efficiency of stimulation of a well. It is carried out by directed unloading of the reservoir on the basis of knowledge of the deformation, strength and filtration properties of specific field rocks. Examination of rock mechanical properties is made by laboratory tests of rock samples at the true triaxial apparatus. The methodic and the results of laboratory studies of rocks and of pilot field works on wells are presented on the example of a specific field. They have shown high efficiency of this approach.

1 Introduction

As the result of mudding while drilling, siltation of a reservoir during the operation, plastic deformation of rocks under the influence of shear stresses, wells are often surrounded by poorly permeable zone. In addition, efficiency of many widespread EOR methods, such as acid treatment, hydropulse impact, is caused by the existence of a good hydrodynamic link between the reservoir and the well.

The permeability of well drained zone can be restored by a simple and effective way – by creating a new artificial system of filtration channel by rock cracking using directional unloading reservoir. This method, known as the "geoloosening method" (Khristianovich et al. 2000), has successfully passed pilot field tests, and is based on the capability of majority of rocks to fracture and to disintegrate in case of the emergence of certain level shear stresses. The required stress state can be created by decreasing the pressure in the well, combined with certain operations, intended priory to form a bottom geometry (more perforation, slot cutting, etc.). The value of pressure drawdown and bottom hole design can be established by the direct physical modeling of the conditions that occur near the well. For this purpose, the Triaxial Independent Load Test System was created at Institute for Problems in Mechanics of Russian Academy f Science. It is a unique research complex that allows using rock specimens to recreate any stress conditions, arising in the reservoir during any process operations in the well and to study their effect on permeability. Tests are carried out on rock specimens, extracted from the particular reservoir core. The facility is equipped with the automatic control system, which allows to set the required loading program and to conduct control as by force and by displacement. Loading trajectories are determined based on calculations: in simple cases, such as simulation of the conditions in the vicinity of the open borehole – analytically, in more complex cases – numerically.

Abstract Carbonate matrix acidization extends a well’s effective drainage radius by dissolving rock and forming conductive channels (wormholes) from the wellbore. Wormholing is a dynamic process that involves balance between the acid injection rate and reaction rate. Generally, injection rate is well defined where injection profiles can be controlled, whereas the reaction rate can be difficult to obtain due to its complex dependency on interstitial velocity, fluid composition, rock surface properties etc. Conventional wormhole propagation models largely ignore the impact of reaction products. When implemented in a job design, the significant errors can result in treatment fluid schedule, rate, and volume. A more accurate method to simulate carbonate matrix acid treatments would accomodate the effect of reaction products on reaction kinetics. It is the purpose of this work to properly account for these effects. This is an important step in achieving quantitative predictability of wormhole penetration during an acidzing treatment. This paper describes the laboratory procedures taken to obtain the reaction-product impacted kinetics at downhole conditions using a rotating disk apparatus, and how this new set of kinetics data was implemented in a 3D wormholing model to predict wormhole morphology and penetration velocity. The model explains some of the differences in wormhole morphology observed in limestone core flow experiments where injection pressure impacts the mass transfer of hydrogen ions to the rock surface. The model uses a CT scan rendered porosity field to capture the finer details of the rock fabric and then simulates the fluid flow through the rock coupled with reactions. Such a validated model can serve as a base to scale up to near wellbore reservoir and 3D radial flow geometry allowing a more quantitative acid treatment design.

Abstract 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 acid, especially at high temperatures. To overcome many of these drawbacks, organic-HF acids have been used as an alternative to mud acid. However, very limited research has been performed to reveal the reactions between organic-HF acids and minerals in sandstone reservoirs. In this study, formic-HF and acetic-HF acids were examined to react with various clay minerals (kaolinite, chlorite, and illite), in comparison with mud acid. A series of acid mixtures with different ratios and concentrations were tested. Inductively coupled plasma (ICP), scanning electron microscopy (SEM) and F nuclear magnetic resonance (NMR) were employed to follow the reaction kinetics and products. Core flood experiments on sandstone cores featured with different mineralogy, with dimensions of 1.5 in. × 6 in. were also conducted at a flow rate of 5 cm/min. The core effluent samples were analyzed to determine concentrations of Ca, Mg, Fe, Si, and Al by ICP. Both formic-HF and acetic-HF acids are much milder than mud acid. The species and amounts of reaction products of different clay minerals in organic-HF acids depend on mineral type, acid composition, and ratio. This conclusion is further confirmed by core flood experiments, in which sandstone cores with different mineral compositions give quite different responses to the same acid mixture. This paper will discuss the detailed chemical reactions that occurred within cores and were followed by chemical analysis of core effluent samples.

Abstract Drilling operations including drilling and completion fluids are the primary source of the reservoir rock damage. Drilling fluid filtrate accumulates around the near well-bore and decreases the reservoir rocks permeability; this can reduce the oil and natural gas flow. Drilling fluid filter cake usually consists of a biopolymer such as xanthan gums, starch along with bridging materials like sized calcium carbonate particles. Polymers are used for improving the carrying capacity of the mud, and the starch along with bridging materials to establish the mud cake on the formation and minimize leak off of the drilling fluid into the formation. Cleaning mud cake is essential for all gravel pack and open hole completion wells, it is important for stimulation operations. Cleaning mud cake maximizes the production, and injection rate by minimizing the formation damage caused by the drilling fluid filtration and deposition across the reservoir rocks while drilling operations. Extensive laboratory studies were conducted to determine the extent of formation damage that occurs while drilling the pay zone section. This paper defines novel methodology involving thermal - gravimetric analysis, to quantify the internal, external filter cake clean up efficiency. The novel lab test can quantify the cleaning efficiency of any breaker; it has several advantages over the conventional methods. This paper demonstrates the benefits and the drawbacks of some breakers (including polymer specific enzymes), aiming to select the best breaker for each well condition and reduce induced damage caused by the drill-in fluid.

Abstract The CaraCara field in the Los Llanos area of Colombia is a highly permeable (1 − 3 Darcy) sandstone with more than 10% total clay content, producing heavy crude. Gravel packing is required to avoid sand production. However, the Productivity Index after conventional gravel packing is only 50% of the open-hole potential due to drill-solids—calcium carbonate —plugging the matrix and fines migration. Non-acid and acid based preflushes have been used to dissolve the drill-solids and fines in the critical near wellbore matrix respectively. However, corrosion, uncontrolled filter cake dissolution, sludging with heavy crude and disposal of spent acid limits the use of acid pre-flushes. A recently developed non-acid based fluid capable of stimulating high temperature sandstone formations was taken as the starting point to develop a customized solution to remove formation damage and control fines migration for this particular application. The base fluid is chelant such as diammonium EDTA (DAE) to which a low concentration of HF acid is added, most commonly in the form of ammonium bifluoride. The HF acid greatly increases the dissolution of aluminosilicates, while the chelant prevents the precipitation of silica gel. The addition of boric acid effectively retard the dissolution rate of the fluid as boric acid the reacts with the HF acid to form fluoroboric acid (HBF4) which slowly hydrolyses to release HF acid. Fluroboric acid also provides an effective means of fines migration control. By adjusting the concentration of acid salts in the fluid as a function of temperature the fluid can provide effective controlled filter cake removal, matrix stimulation and fines migration control, at temperatures between 120 and 300°F. The newly developed non-acid fluid system provides some unique advantages for gravel packing and matrix stimulation applications in highly permeable heavy oil reservoirs 1) A single carrier fluid, eliminating the need for preflushes 2) Controlled filter cake dissolution ensuring circulation is maintained while gravel packing horizontal wells. 3) Effective fines migration control. 4) Scale inhibition. 5) Crude compatibility and negligible corrosion rates. In one field the average productivity index of vertical wells completed using this fluid increased from 0.2 bbl/psi to 0.9 bbl/psi. The final skin values were close to zero and the production remained stable for an extended period of time. A similar fluid when used in horizontal well applications reduced the cleanup time from days to hours. The ability to stimulate while gravel packing optimizes the productivity of dirty sandstones while minimizing the cost.