Summary Several oil wells in a Saudi Arabian field have shown a significant decline in oil productivity in recent years. A few have "died" prematurely, while others have become intermittent producers. The oil productivity decline is aggravated by water encroachment and has occurred with relatively low water rates and without any significant drop in reservoir pressure. These wells have low productivity indices (PIs), resulting in relatively low flowing bottomhole pressures (FBHPs). This paper presents the results of an investigative case study to determine the causes of productivity decline in these wells. A multidisciplinary team was set up with engineers and scientists from reservoir management, production engineering, and the R&D Center for the investigative study. The team focused on multiple aspects, including reservoir and production engineering as well as a comprehensive laboratory and field investigation. The results of this study indicate that one of the main causes of productivity decline in these wells is related to asphaltene precipitation and the subsequent formation of tight emulsions downhole. The emulsions block the pore throats and cause formation damage, which leads to productivity decline. Another factor that further aggravates the productivity of these wells is poor rock quality in the area. Possible causes of formation damage by inorganic scaling and leakage and mixing of gas from a deeper reservoir have been eliminated. Well-test analyses on some of the affected wells show that the formation-damage mechanism in the affected area is further aggravated by poor reservoir rock quality. The time-lapse pressure-transient analysis also indicates a deterioration of skin and productivity with time. On the basis of these findings, a special solvent treatment was recommended and designed as a pilot trial for one of the dead wells. The treatment included squeezing xylene and demulsifier to dissolve the asphaltenes and break the tight emulsions around the wellbore area. The treatment resulted in only a slight improvement in the PI, and the well died after a few days. Currently, a stimulation treatment with acid and demulsifier is being implemented in selected wells. The results of the field trials are described here. Introduction Several wells in the northwestern part of a Saudi Arabian field have shown a decline in productivity in recent years. A few wells have died prematurely at relatively low water cuts, some as low as 25%, which is atypical for wells in this area. It has been noticed that the oil productivity decline is aggravated when wells become wet. The decline has occurred with water rates remaining mostly stable and without any significant drop in reservoir pressure. A location map of affected wells is shown in Fig. 1. Oil and water production rates are plotted in Figs. 2 through 4 for three affected wells. The oil production rates declined from ~ 10,000 to 12,000 B/D to less than 1,000 B/D during a period of approximately 4 to 5 years. Water rates remain generally low, less than 2,000 B/D. It can also be observed that the oil rate decline is substantial as soon as water breaks through in the well. This study was initiated with the objectives of finding the causes of productivity decline in these wells and of finding effective ways to mitigate the problem. A multidisciplinary team was set up with members from reservoir and production engineering and the R&D Center. Several potential causes of productivity decline in these wells were investigated, including the precipitation of asphaltenes, emulsion blocking, mixing of hydrocarbons from a deeper reservoir, inorganic scale precipitation, aquifer-brine and injected-water compatibility, and regional geology, including rock quality, drilling fluid damage, and distance of wells from the gas/ oil separation plant (GOSP). This paper presents extensive experimental work, reservoir-engineering and pressure-transient analysis studies, and results of a field trial. Experimental Investigation Asphaltene Precipitation. Asphaltenes comprise the heaviest polar fraction of crude oils. Asphaltenes exist in the form of colloidal dispersions and are stabilized in solution by resins and aromatics that act as peptizing agents. Asphaltene precipitation and deposition may occur deep inside the reservoir, near the wellbore, and/or in processing facilities.1–4 It was evident from preliminary analyses of wellhead samples that some form of asphaltene precipitation was taking place in the affected wells. All these wells showed tight emulsions, and asphaltenes were observed in the bailer samples. Asphaltene precipitation is a function of pressure, temperature, live crude oil composition, and, to a lesser extent, oil/water interactions. Asphaltenes have a tendency to precipitate as the pressure is reduced, especially near the bubblepoint. However, precipitation can occur even at pressures higher than the bubblepoint, depending on the crude. Normally, this reduction in pressure occurs in the wellbore, where it might not be such a problem because the precipitated asphaltenes may be dragged to the GOSP and redissolve as the pressure reduces further.1,3 However, if the pressure reduction occurs inside the reservoir, for example near the wellbore, it may result in asphaltene precipitation within the effective pore space. This may lead to an increase in skin and, subsequently, more precipitation. Ultimately, this may result in the reduction of oil rates and lead to the death of the well. Asphaltenes are also known to stabilize emulsions.5–8 Tight emulsions can lead to emulsion blocking, a phenomenon that also reduces productivity in oil wells.
Passive Inflow Control Devices (ICDs) are used to enhance performance of horizontal wells in unfavorable environments, such as nonuniform permeability and/or pressure along horizontal sections. This is the first attempt at using ICDs to manage inflow along horizontal wells with substantial reservoir pressure differential. This paper presents how passive ICD completion technology is used to optimize well productivity and reduce water production in a well with significant reservoir pressure differential across the horizontal section. Prior to running the ICDs, this horizontal carbonate well exhibited a nonuniform production profile, cross-flow, and high water production resulting in reduced well performance.A production log of the initial (open hole) completion confirmed downward cross-flow of fluids from heel to toe and production contribution from only the first 10% of the horizontal section. Following the installation of the ICD system, water production was greatly reduced and oil rate doubled indicating significant improvement of the well performance. A second PLT survey showed contribution from the entire horizontal section and elimination of the cross-flow.The key factor in the success of this project was the ICD system design derived from numerous wellbore hydraulic simulations, determining the appropriate number of ICD units and compartments.
Passive Inflow Control Devices (ICDs) are used to enhance performance of horizontal wells in unfavorable environments, such as nonuniform permeability and/or pressure along horizontal sections. This is the first attempt at using ICDs to manage inflow along horizontal wells with substantial reservoir pressure differential. This paper presents how passive ICD completion technology is used to optimize well productivity and reduce water production in a well with significant reservoir pressure differential across the horizontal section. Prior to running the ICDs, this horizontal carbonate well exhibited a nonuniform production profile, cross-flow, and high water production resulting in reduced well performance.A production log of the initial (open hole) completion confirmed downward cross-flow of fluids from heel to toe and production contribution from only the first 10% of the horizontal section. Following the installation of the ICD system, water production was greatly reduced and oil rate doubled indicating significant improvement of the well performance. A second PLT survey showed contribution from the entire horizontal section and elimination of the cross-flow.The key factor in the success of this project was the ICD system design derived from numerous wellbore hydraulic simulations, determining the appropriate number of ICD units and compartments.
Reservoir monitoring is an important aspect of prudent reservoir management to sustain productivity and achieve higher hydrocarbon recoveries. Monitoring is a process that comes in various forms, such as that of flood front advancement and reservoir saturation changes and quantification. Designing and implementing an effective monitoring program to track fluid advancement and quantify remaining oil saturation is a reservoir management best practice that ensures optimum sweep is achieved; and so is crucial for all fields, regardless of their state of maturity. The necessity for such programs becomes more critical as fields mature. Reservoir saturation monitoring programs are usually faced with several challenges, including: mixed and low fluid salinity, tool limitations, borehole conditions and reservoir heterogeneity. Overcoming these challenges requires comprehensive programs that encompass adoption and integration of various derived saturation techniques. This paper will discuss a reservoir monitoring program of a large carbonate field that has produced continuously for several decades. The monitoring program includes "key monitoring wells" in addition to drilling new evaluation wells that are strategically selected and are mostly located in well flooded areas. Time-lapse production and fit for purpose saturation logs are run in the existing wells, while extensive in situ measurements of fluid saturation are collected in the case of the new wells, to monitor saturation changes and track the movement of fluids. The paper will also discuss the various methodologies adopted to address the aforementioned challenges. It will illustrate how the monitoring program has aided in tracking fluid movement, quantitatively determining fluid saturations and assessing sweep efficiency (Ed, Ev and Ea). In addition, the paper will show how the collected information was a catalyst in identifying sweet spots in flooded regions, and therefore guiding development activities for maximizing hydrocarbon recovery, especially from mature areas.
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