Field Testing of the Formation Heat Treatment Process

Abstract

Abstract A novel matrix stimulation concept, formation heat treatment (FHT), which involves the application of intense heat for the treatment of water-blockage and clay-related formation damage in water-sensitive formations was developed and tested in the laboratory. Initial bench-scale heating tests on water-sensitive sandstone cores showed that heat treatment at 600 ° C can improve air permeability of a damaged core by 50% above the initial permeability. Dramatic permeability increases of 750% and 1,000% above the initial reservoir permeability occurred at 800 ° C for the cores taken from the gas- and oil-bearing formations, respectively. To prove the FHT technology in the field, an electrical downhole heater was designed and constructed. After testing the heater on surface, the heater was lowered into the target reservoir, 1.5 km downhole, heated up and retrieved from the wellbore. Results of the field test showed that there was a significant increase in the post heat-treatment gas injectivity. To quantify the heating effect, pressure transient analyses on pressure falloff data were carried out and the post heat-treatment permeability was found to increase by several fold. The field logistics and the field test results are presented in this paper. Introduction Most hydrocarbon-bearing (oil and gas) sandstone reservoirs contain clay materials, which have the tendency to swell in contact with water-based foreign fluids or to migrate due to fluid velocity in the pore spaces. Operations such as drilling, completion, workovers, and stimulation often expose the formation to a foreign fluid and cause formation damage. Such damage is usually more severe in horizontal wells, because of the longer exposure of the wellbore to the offending fluids(1). During the drilling and completion phases, the primary mechanisms of near-wellbore formation damage can be explained by the following factors:Pore-throat constriction, caused either by clay swelling due to incompatible fluids or by clay migration,Water blocking due to reduction in relative permeability to hydrocarbon,Plugging with drill solids and mud products, andLoading of the reservoir with drilling or completion fluids. Formation damage during drilling and completion has long been identified as a major problem. Preventive measures to stabilize clay swelling and migration, mostly consisting of the use of various chemicals (e.g., KCl) in the drilling or completion fluid, have been discussed in the literature(2-7). However, prevention of clay damage is not always possible or effective, and curative measures may then become necessary. Several curative methods have been attempted and presented in the literature(8-15). The most popular stimulation technique is hydraulic fracturing. This method is not always applicable or desirable (e.g., in water or gas-flooding situations, zones containing active bottom water or gas caps) or not economical (e.g., in some horizontal wells). Another approach is to stimulate the near-wellbore region using acids, which dissolve either the clay minerals themselves (HF acid) or the surrounding formation rock (HCl and HF acids). Matrix-stimulation techniques using acids have been applied to carbonate reservoirs for productivity improvement. In both hydraulic-fracturing and acid-stimulation techniques, the proper design of the fluid system is very important.