Summary Heavily clay-laden, low-permeability sandstones have always presented difficulties in stimulation. The most common treatment for such presented difficulties in stimulation. The most common treatment for such formations has been a crosslinked 40/60-lbm hydroxypropyl guar (HPG) in KCl water, but the results have not always reached expectations. This paper characterizes one of these formations by scanning- electron-microscope/energy-dispersive-spectrometer (SEM/EDS) evaluation mineralogy and lithology. The core samples were subjected to conventional and experimental treatment fluids by a nitrogen gas reference core-flow apparatus to find the optimum stimulation treatment. Introduction Core samples of the Codell sandstone formation taken from several wells in the Denver-Julesburg basin were used for the majority of this work. Fluids evaluated by core flow were KCl-water-based systems, weak acid blends, KCl-water/methanol blends, and 100%-methanol and low-ph methanol systems with specific additives. One low-pH methanol system developed during this study was shown to be an effective enhancement of the conventional water-based systems currently used in the treatment of tight, dirty sandstones. The success of a hydraulic-fracturing system in the treatment of sensitive, clay-filled sandstones depends on its ability to place sufficient quantities of proppant without inflicting damage to the matrix of newly created fracture surfaces. The problems encountered in treating these formations that will be addressed in this paper are (1) permeability damage to fracture faces, (2) retention of permeability damage to fracture faces, (2) retention of treating fluid by reservoir rock, and (3) water blocking. The Codell sandstone is a difficult formation to treat. Conventional crosslinked 40/60-lbm HPG in KCl- water-based systems (usually with pH 7 to 9) make up approximately 95 % of all the treatments of the Codell to date. Rarely is more than 30% of the treating fluid recovered after proppant is put away; the average recovery is 20%. Some of this fluid loss can be attributed to fracturing out of zone. The rapidly declining production curves often seen in wells in this formation, however, suggest the possibility of water blocking. possibility of water blocking. Three distinct types of Codell were characterized by SEM/EDS analyses, and each type was then subjected to core-flow treatment with a number of fluid systems:two commonly used KCl-water-based systems at pH 4 and 7,10 weak-acid blends,100% methanol,2% KCl-water: methanol (50:50), andfive low-ph methanol systems. These systems are identified in Table 1. These core-flow tests show the effects of each system on the permeability of the sample rock. One system, LMX 02, shows the most consistently positive results in permeability improvement of not only the Codell but other permeability improvement of not only the Codell but other similar sandstones as well. When used as a prepad treatment, this new system dramatically improves the performance of conventional KCl-water-based systems in performance of conventional KCl-water-based systems in enhancing the permeability of the Codell and other sandstones.
The emphasis on providing protection to sensitive clays in a variety of reservoir types has led to the development and application of specific chemistries. These range from monovalent to polyvalent salts and more recently, to polymer based systems. In numerous published cases exotic interaction studies have been performed to evaluate the potential effectiveness of these clay protection additives. The need for a simple standard screening technique to allow comparative testing and ultimately specific designs for clay additive usage is obvious. This technique will illustrate the significance of clay species interaction with clay stabilizing fluids and point to any harmful side effects on reservoir permeability that may be caused by the clay stabilizer. The study presents a simple procedure for indexing clay stabilizing additives in order to insure that the most effective one is chosen and implemented as a "specific" agent to control a "specific" clay situation. Use of a packed column in addition to berea core allows the designer to vary the type and amount of clay in the column in order to simulate the clay content of the formation in question. Factors affecting the performance of clay stabilizers will be prioritized. They include compatibility with other additives, permeability damage, and ability to prevent clay migration in a packed column. An attempt will be made to document results with respect to clay chemical/clay species interaction. This will be done before and after permeability flow tests using scanning electron microscopy. The evaluation of stabilizers examined in this study will be documented by indexing each chemical. This index will be shown to be a useful tool in differentiating among the numerous clay stabilizers available in the market today.
SummaryAn automatically pressurised infusor system, the Alton Dean infusor, has been compared with two types (cloth and plastic) of commonly used pneumatically pressurised bags. All the infusors had reasonably accurate pressure gauges but pressure could only be consistently maintained with the cloth infusors or with the Alton Dean infusors when connected to a compressed air supply. Sequential fast infusions were possible with all infusors, but simultaneous infusions could be limited in number when the pressure infusor was connected directly to pipeline gas. One of the Alton Dean pressure infusors tested had a leak in the pressurisation system, and the pressure adjustment valves were dficult to manipulate. These may require modijcation.
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