With the passing of "easy oil," the need for high-pressure, high-temperature (HP/HT) drilling and completion fluids has increased. This has, in turn, increased the need for sufficiently robust cleanup methodology. This requirement for high density, solids free cleanup fluids drove the investigation into the use of cesium formate as the base brine. The use of cesium formate brings unique challenges to the cleanup, most notably, the cost implications. The high price of cesium formate brine means that any losses could severely impact upon the cost of the cleanup operation. Furthermore, any brine used in cleanup pills that could not be recovered for re-use would have to be considered as lost. It was, therefore, imperative to return the brine to its original condition, thereby avoiding the need to dispose of a very expensive commodity. This paper will look at the laboratory testing surrounding the development of a high density cleanup fluid (HDCUF) and the ability to remove the contamination from the brine to bring it back to an acceptable condition.The cleanup spacers were made up with cesium formate brine and several approaches to remove the contamination were investigated. Contamination was from both the chemicals used to formulate the cleanup spacer and from the oil based drilling fluid that would be used to drill the wells. Nuclear Magnetic Resonance (NMR) Spectroscopy was used to measure the level of contamination in the brine.The results of the study showed that, using the method selected, it was possible to remove the cleanup chemicals to an acceptable level where the cesium formate brine was able to be re-used. This would enable the operator to recover the brine and avoid the significant costs from losing the volume and disposing of the waste.
An ultimate objective is specified to overcome the oil productivity deterioration in a cased hole producer that was drilled throughout by means of oil-based mud (OBM). Significant problems can be caused by OBM invasion into the reservoir and/or contamination by the OBM interacting with completion/formation brines in the hole. In this study for designing an optimum remedial program, a series of laboratory works were conducted using reservoir cores to understand accurately the damaged conditions/location at the site. For this study of a cased hole well drilled throughout by OBM, an unexpected low oil productivity was assumed with non-negligible water production. Multiple reasons were considered such as near-wellbore formation damage (including wettability change, emulsion blockage, etc.) or plugging in certain sections of the cased hole. To identify the main reason, return permeability tests (RPTs) were performed for several contaminating combinations: OBM versus formation/completion brines, or reservoir oil. Comprehensive evaluation determined the mechanism that caused the problem. Simultaneously, a bottle-test (BT) was conducted to evaluate if a micro-emulsion wellbore remediation fluid would dissolve the contaminated material. The RPTs indicated that minor OBM invasion occurred for the formation-brine-saturated core (RPT-1), and significant invasion for the core saturated with base oil (RPT-2). In the RPT-1 core, most OBM was trapped at the inlet-side core. This reduced the invasion, and the return permeability could maintain 93% of its original value. In contrast, the inlet/outlet surfaces of the RPT-2 core were cleaner than those of the RPT-1 core. This revealed that the OBM had invaded the RPT-2 core. Consequently, the return permeability dropped down to 32% of the original value. The BT showed that the multifunctional single-phase emulsion fluid could dissolve the sticky, contaminated material caused by mixing OBM and the completion brine. The contaminant strongly adhered to the glass tube wall and was quickly detached, creating flocs that could not be removed by solvents such as toluene. Contact angles on the tested cores were measured to understand measured area. These findings were useful for optimizing a remedial program: a two-staged operation consisting of removing contaminants from the cased hole followed by soaking for dissolving the invaded OBM near the wellbore. This study provides the valuable findings of laboratory evaluation of OBM-related damage in the cased hole well. The clarification of problem mechanism and location could be essential data for appropriate remedial design. A single test that screened effective dissolution-fluid was a part of remedial job design, but not enough for optimizing the design. This comprehensive evaluation approach of the RPT and BT demonstrated the most likely bottomhole/near-wellbore situations to be taken into account.
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