Asphaltene Precipitation in High Gas-Oil Ratio Wells

Abstract

Asphaltene deposits have been observed in a number of high gas-oil ratio (GOR) wells in north Ghawar. Even though the oil reservoir is undersaturated two small gas-caps are present as a result of gas injection during the 1960s and 1970s. New development wells drilled recently to produce oil and gas from the gas-cap areas have experienced asphaltene deposition. The cause of precipitation is the stripping of the asphaltenes from the crude by the gas. This paper describes the results of an investigative study that was initiated to determine the precipitation mechanism and ways to alleviate the deposition problem. Asphaltene precipitation experiments were conducted at reservoir conditions in a special PVT apparatus. The effect of gas-oil ratio on asphaltene precipitation was determined by titrating the reservoir oil with gas-cap gas. Bulk deposition tests were also performed at different GORs with reservoir fluids. The results demonstrate that the onset of asphaltene precipitation occurs at relatively low GOR values. However, the amount of asphaltene precipitated at the onset is negligible. Asphaltene precipitation and deposition increase with increasing GORs. Asphaltene deposition envelopes are provided for the reservoir oil as a function of pressure and temperature. Guidelines are provided to alleviate the problem by controlling the GORs. Recipes for solvent treatment including asphaltene dispersants are also described in the paper. Introduction Ghawar field is one of the major oil fields in Saudi Arabia. In the northern part of the field some wells have experienced solid built-up in the wellbore. Analyses of solid samples from several wells have shown the presence of asphaltenes that may have precipitated during crude production and have started to deposit on the wellbore. The solid deposition has been observed in high gas-oil ratio wells. A location map of the wells is shown in Figure 1 and photographs of the solid deposit from one well are shown in Figure 2. The Arab-D reservoir of Ghawar field contains an undersaturated light oil. The bubble point pressure is ~1,900 psi at the reservoir temperature of 215°F and the average gas oil ratio is ~570 scf/stb. The reservoir pressure at present is over 3000 psi. In the 1960s and 1970s the associated gases from part of the field were injected back into the reservoir at two locations due to unavailability of gas processing facilities and to avoid excessive flaring. The injected gases have formed two separate gas caps in the field (north and south gas-caps, Figure 1). In recent years oil production has started from these gas-cap regions. Due to the presence of the gas cap, some of the free gas flows into the oil production wells increasing their total gas oil ratios (GORs). The coning or cresting of gas into the oil has caused limited plugging in a few wells in the north and south gas-cap areas. The gas strips the oil of asphaltenes which precipitate and deposit in the wellbores. Plugging of the wellbore by asphaltenes or organic deposits has the potential to reduce productivity and cause production impairment. Furthermore, several more gas-cap wells are being planned to be drilled in the area and their productivity may be impacted by the deposition tendency. Figure 3 shows the GOR for 11 wells in which asphaltene deposits have been observed. The solid line shows the average GOR for the entire field (~570 scf/stb). Except for one well, the GOR for all wells is higher (in some cases substantially higher) than the average field GOR. The high GOR is a consequence of gas coning/cresting in the wells. The free gas strips the asphaltenes from the crude which deposit in the wellbore. One well was tagged over a period of time to ascertain the buildup of asphaltenes in the well. Figure 4 shows the tag depths and indicates a loss of wellbore accessibility of ~200 ft over a period of 18 months. Recent results show that the buildup has stabilized and the asphaltenes may be dragged with the oil to the gas oil separating plant (GOSP). Other wells are also being monitored and have shown some buildup activity.

Ready for Gas Injection: Asphaltene Risk Evaluation by Mathematical Modeling of Asphaltene-Precipitation Envelope (APE) With Integration of All Laboratory Deliverables

“…Asphaltene problems have been treated mechanically, chemically, and/or operationally. Among available remedies in each technology that have been widely applied in the world (Jamaluddin et al 1996;Avila et al 2001;Kokal et al 2003;Wang and Civan 2005;Hamid 2006;Al-Ghazi and Lawson 2007;Frost et al 2008), the most conventional ways have been applied in our target fi eld. The selection of remedial action was based on operational and cost reasons.…”

“…The increase of asphaltene risks through gas injection has been recognized as a general rule (Kokal et al 2003;Srivastava et al 1999;Parra-Ramirez et al 2001;Sarma 2003;Jamaluddin et al 2000;Kokal et al 2004;Negahban et al 2005;Okwen 2006; Moghadasi et al 2006). A preparatory evaluation of asphaltene behavior under gas-mixing conditions is important to minimize risks and to optimize the gas-injection project.…”

Chemical changes of fluid inclusion oil trapped during the evolution of an oil reservoir: Jabiru-1A case study (Timor Sea, Australia)

“…One inclusion within a quartz overgrowth in the 1610e1613 m contains a yellow fluorescence oil with brown solid organic matter (bitumen) and has a Q G/B of 1.57, the largest amount of CO 2 (around 10 mol%), the lowest methane content (around 5%) and a high CH 2 /CH 3 ratio. They are interpreted as being produced by the deasphalting of the irreducible oil during the accumulation of the gas, a process commonly observed in other petroleum reservoirs (Milner et al, 1977;Lomando, 1992;Hwang et al, 1998;Kokal et al, 2003). Bitumen, also observed between detrital grains in Zone 1 (Fig.…”

Investigation of Asphaltene Deposition and Precipitation in Production Tubing

“…They used the deposition data from aerosols (deposition rates for aluminum and iron particles in air) to verify their model. Kocabas et al [22] Kokal et al [23] described the results of the study on oil samples of north Ghawar reservoir that started to show precipitation problem and ways to alleviate the deposition problem. They reported that the precipitation was linked to gas coning/cresting in the wells.…”