Productivity Decline in Oil Wells Related to Asphaltene Precipitation and Emulsion Blocks

Abstract

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.

Multifunctional Evaluation of a New Supramolecular Complex in Enhanced Oil Recovery, Removal/Control of Organic Damage, and Heavy Crude Oil Viscosity Reduction

“…However, the colloidal instability index is slightly greater for this light crude oil sample, indicating a higher asphaltenes instability 3 . On the other hand, light crude oil sample of the JU system with a known tendency for asphaltenes precipitation during production operations presents, as expected, a considerably higher colloidal instability index as can be observed in Table 1, indicating an alkane-rich and hence asphaltene-hostile environment 3 . Composition of the high salinity brines used for the PR, TC and JU systems can be found in a tabular form (Table S1) as Supporting Information.…”

“…Asphaltenes can deposit both in the producing slots and in the formation face. Deposition of asphaltene precipitates over pore surface and/or across pore throats may reduce the effective pore space and/or severely impair the ability of oil to flow through porous formation, altering the rock wettability from water-wet to oil -wet, and increasing hydrocarbon viscosity by nucleating water-in-oil emulsions [1][2][3][4][5] . On the other hand, enhanced oil recovery (EOR) processes have become increasingly important to the petroleum industry.…”

Removal of Plugged Asphaltene in a Reservoir by Earthquakes

“…The severe asphaltene problems of Field 'M' are similar to those of other cases in which asphaltene problems were reported (Leontartis and Mansoori 1987;Garcia et al 2001;Kokal et al 2003;Strippoli et al 2010;Al-Qattan et al 2012). However, a unique feature of this case is that frequent earthquakes have hit this field, and the reservoir has responded positively to certain earthquakes in terms of oil production rates and flowing wellhead pressures.…”

“…The problems related to asphaltene deposition in Field 'M' are presumably comparable with those reported in the literature (Leontartis and Mansoori 1987;Garcia et al 2001;Kokal et al 2003;Strippoli et al 2010;Al-Qattan et al 2012). However, a unique feature of this field is that it has been struck by frequent earthquakes, and the reservoir has responded to certain earthquakes.…”

“…Fig. 4 shows that the CIIs of both Well A and Well B were comparable or higher than those from other fluids from locations in the Middle East (Kokal 2003;Misra et al 2011;Kabir and Jamauddin 2002;Al-Qattan et al 2012) and Venezuela (Garcia et al 2001) that have experienced asphaltene problems in their production systems.…”