May 2010 ISSN

Transcription

1 May 2010 ISSN

2 By M. Samir, W. Hassan, Scimitar Y. Abugren, S. Joshi, E. Thabet, Schlumberger Reservoir Characterization of the Issaran Heavy Oil Field The Key to Economic Development Heavy crude oils, those with API gravities less than 22 o provide about 12% of global crude supplies today. 1 A significant portion of this comes from non-conventional extra-heavy crude in Venezuela and bitumen in Canada, with API gravities <10 o. By 2020, heavy oil will make up over 20% of crude supplies and 16% of global liquid supplies. 2 Conventional heavy oil (API gravity >10 o and < 22 o ) has been produced in sandstone reservoirs in California, China, Indonesia, and Latin America for decades using both cold production and steam Enhanced Oil Recovery (EOR) techniques. Production of bitumen and extra-heavy crude in Canada and Venezuela is more recent, made possible by advances in technology and higher oil prices. Though most heavy oil today comes from sandstone reservoirs, there are huge discovered heavy oil resources in carbonates, especially in the Middle East, where some 500 billion barrels original oil in place are known to exist. Much of this oil is within the API gravity range that would be considered conventional heavy oil if it were located in sandstone formations. However, carbonate formations are complex with widely varying permeability and porosity within the same formation and they are often naturally fractured. Only a few heavy oil carbonates are being produced, including pilot steam injection projects in the Neutral Zone between Kuwait, Saudi Arabia, and Oman Another is the Issaran field in Egypt where unique application of technology has improved oil production and enhanced the viability of this complex carbonate field. Issaran Field Description and History The Issaran field contains heavy to extra-heavy oil with an API gravity of 10 o -12 o API. The fractured carbonate reservoirs contain about 700 million barrels oil in place in three main formations (Figure 1). The Upper Dolomite formation is a fractured dolomite 140 feet thick at a depth of 1,000 ft. Reservoir temperature is 120 o F and pressure is as low as 250 psia. The Lower Dolomite is similar to the Upper Dolomite. The oil in these zones is too viscous to be Figure 1: Nukhul well with high intensity fracture extracted using cold production techniques. The deepest zone is the Nukhul, a highly fractured limestone with 30% porosity and three to 300 md permeability that is the source of most of the cold production. Issaran was discovered in 1981 and began producing in 1998 under a joint venture between Egyptian General Petroleum Corp. (EGPC) and Scimitar Production Egypt Ltd. At that time the recovery factor was projected to be less than 1%. Well rates were very low, less than 30 bfpd; total field production was 170 bopd. More wells were drilled; the production was 2,000 bopd in Jan 2006, the year when EGPC s interest was acquired by Rally Petroleum of Canada, a company with experience in the Canadian oil sands. Rally sold their interest in 2008 to National Petroleum Company of Egypt and Scimitar took over as operator. Current production is 4,500 bopd (March 2010). All figures are courtesy of Schlumberger 1 Atkins, Laura and Higgins, Terry, Heavy Crude Oil Outlook: Global Analysis ; Hart Energy Consulting Ibid. Petroleum Africa May

3 Figure 2a Figure 2b Figure 2c Figures 2a and 2b: ISS-85 flowing survey at 70 HZ (left), ISS-85 flowing survey at 80 HZ, Figure 2c: ISS-85 shut in survey Steam assisted gravity drainage (SAGD), considered to be the state-of-the-art in steam EOR, is not applicable to the Issaran field. SAGD is effective in thick, relatively homogenous reservoirs with high vertical permeability. Although they have good porosity and are highly saturated with heavy oil, the producing zones in Issaran contain impermeable stringers that create breaks in vertical permeability. Thus, Rally and Scimitar decided to try cyclic steam stimulation (CSS) beginning in 2007 with four CSS cycles in a pilot well. The results were favorable and steam injection was expanded. By mid-2007, the field was producing over 6,000 bopd. Different completion practices had been developed for each zone. The Nukhul formation wells had been completed open hole. Upper and Lower Dolomite formation wells were completed as cased and open hole steam injection wells, producing by cyclic steam stimulation. Water production was excessive in all zones. Field Evaluation and Optimization The Issaran field presents unique challenges to field development and operations, and as such was under-developed for many years. Rally and Scimitar contracted with the Schlumberger Data and Consulting Services (DCS) team to identify opportunities to improve production. These opportunities ranged from changing drilling programs, optimizing stimulation, and identifying the source of high volumes of water production. When the original wells were drilled, circulation was being lost almost immediately as the bit reached the reservoir. The drilling crew would start pumping down various lost-circulation materials, which were plugging the fractures, permeability of which is responsible for the reservoir productivity. It made little sense to try and plug the formation. Based on advice from Schlumberger, the drilling crews switched to treated water prior to drilling into the reservoir. There is no risk of a blowout with the heavy, viscous oil, and the treated water can be produced out easily without formation damage. The DCS team identified opportunities to improve production through optimized stimulation. Wells were acid-fractured and there was an immediate improvement in production rate. In some wells, these induced fractures simply healed themselves and production soon fell off. In such cases, Schlumberger used the FracCADE * fracturing design and evaluation software to design propped fractures. The fracced wells production increased by 40%-300% per well. Production Logging Excessive water production continued to be a problem in all zones; thus Scimitar, the current operator, decided to run production logs to identify the sources of the water and to locate fractured, high permeable layers. In the Nukhul, because of the high fracture density (Figure 2), water rapidly channeled through the fractures and overshadowed oil production. The objective of the production logs was to identify the source of water, evaluate the flow contributions from the different zones and determine whether or not there was cross flow in the reservoir. Hence, a high drawdown electrical submersible pump (ESP) (4,000 bfpd) was required in order to produce the oil zones. Since the available y-tools had a rate limit of 1,000 bfpd, a special y-tool with high capacity ESP was designed for Scimitar by Zenith to log the wells under high flow rate dynamic conditions (Figure 3). Use of the Schlumberger 400 series ESP in conjunction with the new y-tool, enabled Scimitar to reach the targeted well production rate. This was the first application of the y-tool. In the Upper and Lower Dolomite formations, more than 100 wells had been completed by the time the production logs were run, initially as open hole oil producers. Layers within the wells had varying pressures. After initially producing oil, a sudden increase in the water cut was observed in most of the old producing wells and in many of the new wells. The low salinity of the produced water (<10,000 ppm) made it difficult to identify the source of the water. Because of the similarity between the oil and water densities, another tool, other than the density tool, had to be used to differentiate between water and oil. Thus, the production logging tool string included the FloView * holdup measurement tool. FloView measures the conductivity of the fluid at the tip of the probe (Figure 3). Petroleum Africa May

4 production in vertical open-hole completions, since the wells initially produced high liquid rates, but the super-k layers soon channeled water into the wellbore. Injected steam also channeled through the fractures into the water zones. Figure 3: Nukhul Production Performance Other tools used were pressure and temperature sensors, and fullbore and inline spinners. Tool String and Risk Mitigation The y-tool had to be built for 7" casing with the optimum tubing size for production. The minimum restriction was 1.768" with a drift diameter of 1.698". The recommendation from the y-tool provider was not to run anything larger than the drift diameter, ruling out the use of tools such as the RST * reservoir saturation tool (in WFL * water flow log mode to identify water production. This is illustrated by the result from ISS-85 well. This well had been completed as an open hole cold oil producer. The water cut rapidly increased to 98% water cut at a total fluid rate of 6,000 bfpd, and continued to increase, reducing the oil production to 20 bopd. Two flowing PL surveys at different flow rates and one shut-in survey were run. Based upon the measured bottom hole pressures during the three surveys and the interpreted production rates, four reservoir zones could be identified within the open hole interval of 140 ft. It was found that only about 30 ft of the interval was contributing the small amount of oil production at high drawdown. The remaining zones were producing water, two of which were the main source of the water. The flowing survey results are shown in Figures 4a and 4b. During the shut-in survey PL sensors detected a clear cross flow of water into the topmost zone from the other intervals below as seen in Figure 4c. There were risks of the tool string becoming stuck. Fishing operations would have been difficult to impossible because the tool string would be sitting on one side in the 6" open hole section. Thus, a centralizer was placed below the logging head. The viscous heavy oil could potentially stick on the FloView measurement tool probes and on the spinner. Although the water cuts in the wells were high, which would reduce this risk, it could not be completely controlled and so the decision was to accept this risk during the operation. The small clearance between the tool string outer diameter, 1.688", and the inner diameter of the y-tool completion, 1.768", posed a potential deployment challenge. To help ensure service quality and to avoid surprises on the job, the y-tool was brought to the Schlumberger base to run the tool string through the completion in horizontal conditions in a dry run to confirm that it could pass through the restriction. It was able to do so, and an entry guide was connected to the bottom of the completion to ensure that once it left the tubing it could be retrieved. Production Logging Results Production logging (PL) was successfully carried out in three Issaran wells. In the two Nukhul wells, high permeability (super-k) layers were identified and their pressure and productivity index were calculated. In the Lower Dolomite well, water was found to be produced from layers having resistivities up to 30 ohm-m. This provided a new resistivity cutoff to identify hydrocarbonproducing zones. The PL results significantly influenced future development plans. Scimitar had initially planned to develop the Nukhul with cold Figure 4a and 4b: Production profile, CSS-342 cased hole vs CSS-323 open hole (left). CSS-355 cased hole vs CSS-346 open hole (right) The calculated flow profiles from the PL surveys matched well with the flow rates measured at surface, indicating good agreement with the survey results. As a result of the PL logs, a 5" liner was installed and cemented in the open hole interval in this well and the water zones isolated. Oil rate went from negligible to about 70 bopd. Increasing the drawdown in the Nukhul formation was found to increase oil production; producing the Nukhul with the new ESP increased production up to 20,000 bfpd. This had the favorable side effect of decreasing the water cut in some of the surrounding wells (Figure 5). In the Upper and Lower Dolomite formations it was decided to complete some newly drilled wells as cased hole producers as a pilot study, to compare the trade-off between shutting off the water production and the effect of the additional pressure drop through the Petroleum Africa May

5 of production logging tools through a special high-rate y-tool provided valuable data on sources of water channelling and steam breakthrough, and identified water-bearing zones based on a resistivity cutoff. This information led to a change in field development strategy from one Figure 5: Liner strategy perforated completion upon the productivity. A comparison between the open and cased hole completion results are illustrated in Figures 6a and b. The cased wells produced oil while hardly any oil production (green curve) is seen from the open hole wells (lower graphs). The new development plan calls for existing open hole wells to have 5" liners installed then converted to CSS. High oil saturation zones will be selectively perforated for steam injection (Figure 7), avoiding highly fractured zones. The idea was tested in wells CSS-86 (Upper Dolomite) and well CSS-121 (Upper Dolomite). As shown in Figure 8, incremental oil production is about 40 bopd from each well after installing the liner and selectively perforating the oil interval. Conclusions Issaran is one of the first fields in which steam EOR has been successfully implemented in a heavy oil carbonate reservoir. The use Figure 6: CSS-121 & 86 before and after the liner based on open hole completions to the use of cased wells and selective perforating within oil zones. The information also ensured that steam was not injected into highly fractured zones. Results from test wells confirmed that the new completion strategy significantly increased oil production. *Mark of Schlumberger Petroleum Africa May