Reservoir Pathway Identification in a Fractured Carbonate Heavy Oil Reservoir

The Issaran field, situated in the Egyptian Eastern Desert, is one of the few heavy oil fractured carbonate reservoirs in the world. It has an estimated 700 MMBBLS of 10-12 degree API crude in the Upper and Lower Dolomite reservoirs and the deeper Nukhul reservoir with 10% H2S. Production in 2008 was 5000 STBOPD, forecasted to increase to 6000 STBOPD in 2010. A Cyclic Steam Stimulation (CSS) project was started in Issaran in 2006.

Heavy oil is an unconventional oil resource that is characterized by higher viscosities and densities as compared to conventional oil. Most of the heavy oil reservoirs are deposited at very shallow depths. These hydrocarbons originated as conventional oil that formed in deep formations, which then migrated to the surface regions where the lighter hydrocarbons escaped.  Primary recovery from such unconventional reservoirs can be as low as 1%, with ultimate recovery generally being < 30%.

Issaran oil field is located 290KM southeast of Cairo and 3KM inland from the western shore of the Gulf of Suez covering an area of 20,000 acres. The field was discovered in 1981 (Figure 1). The heavy oil project started in 1998 between GPC (General Petroleum Company) and Scimitar Production Egypt Ltd. The OOIP at that time was 410 MMBBL, reserve 0.2 MMBBL, recovery factor below 1% and the daily production was 170 BOPD. The productivity of the wells was very low and the average production per well was below 30 BOPD.

The field has been producing from different zones and each zone has its own unique characteristics. This required a special approach to overcome potential problems and optimize the production and reserves. The major heavy oil accumulations occur within shallow Miocene Dolomites and Limestones (Upper Dolomite, Lower Dolomite, Gharandal and Nukhul Limestones) and sandstones (Zeit). The average gravity of oil from all zones is between 10 to 12 API.

Figure 1 Issaran Field location

Figure 1 Issaran Field location

The Upper Dolomite Formation is this field is characterized by a depleted fractured dolomite reservoir with 10-12 API heavy oil. The top of reservoir depth is around 1000 ft with an average reservoir thickness of around 400 FT.  The formation pressure is as low as 250 psia with a temperature of 120 degrees Fahrenheit. The oil has 10% H2S content and a viscosity of 4000 cp at standard conditions. The reservoir rock is oil-wet, which is unfavourable for oil recovery factor. The Lower Dolomite formation has similar characteristics as the Upper Dolomite. Due to the shallow depth of the Upper and Lower dolomites, the oil cannot be extracted using cold production. Considering the above characteristics, the exploitation of these formations is very challenging.

The Gharandal formation consists of three limestone bodies with permeabilities lower than 20 mD. The deepest formation is the Nukhul zone, which has been the major producer of cold oil due to its highly fractured nature.

These different reservoir characteristics led to the application of different types of reservoir management for each zone (Figure 2). The Nukhul formation wells have been completed as openhole, Gharandal formation wells have been completed as cased hole and Upper and Lower Dolomite formation wells have been completed as cased /open hole steam injection wells, producing by cyclic steam stimulation.

Scimitar had planned to develop the Nukhul with cold production in vertical open-hole completions, since initial wells gave high liquid rates, due to high fracture permeability. However, these super-K layers soon created unwanted water channeling to the wellbore resulting in high water cuts. Also, injected steam rapidly channeled through the fractures and hence the injected heat could not be retained in the reservoir. Thus the heavy oil viscosity could not be reduced. Viscosity reduction is key to increasing heavy oil recovery.

Figure 2 Issaran Field Formation

Figure 2 Issaran Field Formation

Two main factors were found to contribute to the high water production from different reservoirs. These were water/steam channeling through fractures and difficulty in determination of the correct pay resistivity cutoff. These factors affected the different reservoirs as follows:
1) From Nukhul formation
Due to the high fracture intensity (Figure 3) water rapidly channeled through the fractures and overshadowed the oil contribution.  Hence, an ESP which could create a high draw down was required in order to exploit the production from the oil zones. However, the limitation of the currently available Y-tools in the market represented a barrier to achieve this target. These Y-tools had a rate limit of 1000 BFPD with the available pumps, whereas the actual rate needed for achieving Scimitar objectives was 4000 BFPD. Use of the Schlumberger 400 series ESP in conjunction with the new Y-tool, enabled Scimitar to reach the targeted well production rate.

2) From Upper Dolomite & Lower Dolomite Formation
More than 100 wells were completed in these formations, initially as open hole oil producers. The wells had varying pressures in the open hole layers and consisted of both cold and hot producers. After initially producing oil, a sudden increase in the water cut was observed in most of the old producing wells as well as the new drilled wells. Due to the low salinity of the produced water (< 10,000 PPM) the identification of the source of water was difficult. Hence, determining the correct cutoff value of resistivity to distinguish between water and oil zones was a major challenge. This issue led to a misunderstanding of the correct oil potential in the open hole section.

Accordingly it was urgently needed to identify the water source in order to maximize the oil production as much as possible. An ESP capable of producing very high drawdowns was required to produce the well while performing production logging in flowing conditions, in order to differentiate between the water zones and oil zones in this type of heavy oil fractured carbonate reservoir (Figure 4).

Furthermore, breakthrough of the injected steam had been noticed recently in the field through the super-K layers, which highly reduces the efficiency of the steamflood project. Hence, identification of these super-K layers was essential in order to shutoff the water and steam breakthrough.

Figure 3 Nukhul well with high intensity fracture

Figure 3 Nukhul well with high intensity fracture

It was critical to diagnose these super-K layers in a dynamic condition. Scimitar designed a special Y-tool in association with Zenith, which would enable PL to be carried out under ESP operation with high rates.  The y-tool had to be built for 7” casing and the tubing size was designed to optimize 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 size. This ruled out the use of some tools such as the Reservoir Saturation Tool (RST) in Water Flow Log (WFL) mode to identify the water production.

Considering the similarity between the oil and water densities, another tool, other than the density tool, had to be used to be able to differentiate between water and oil. The proposed tool string consisted of pressure and temperature sensors, fullbore as well as inline spinners and electrical probes (FlowView). Flowview measures the conductivity of the fluid at the tip of the probe, hence differentiating between water and oil. Figure 5 shows the production logging toolstring that was run and Figure 6 shows the operation of the Flowview tool.  Figures 7 and 8 show the details of the new y-tool system.

For the sake of comparison and testing, the density tool was added to the tool string in the first well to check the reading and based on the results a decision would be made on whether to continue using the density tool in the other wells, or removing it from the tool string.

Going through a risk assessment, there were concerns regarding getting stuck and having to break the weakpoint. In this case, the fishing operation would have been difficult to impossible, since the tool string will be sitting on one side in the 6” open hole section. Therefore, a centralizer was placed below the logging head, to ensure that in case of a fishing operation, the tool string would not be lying on one side.

The heavy oil imposed a risk on the tools as well, since it is very viscous and could stick on the Flowview probes as well as sticking on the spinner and restricting the measurements. Although the water cuts in the wells were high, which would reduce the risk of plugging the probes or the spinners, the risk could not be controlled. Therefore, this was a risk that had to be taken during the operation.

Figure 4 Open hole v.s Cased hole

Figure 4 Open hole v.s Cased hole

Due to the clearance between the tool string outer diameter, 1.688” and the inner diameter of the y-tool completion 1.768”, there was a risk that the tools would not be able to go through the completion. To avoid having issues or surprises on the job, the y-tool was brought to the Schlumberger base and the tool string was run through the completion in horizontal conditions, to confirm that the tool string could pass through the restriction. This dry run was performed and results showed that the tool string could pass through the restriction, but once the tools went out of the tubing, there was difficulty in retrieving the tools back into the tubing. Therefore, a wireline entry guide was connected to the bottom of the completion and that resolved the issue.

Figure 5 Wireline Production Logging Toolstring

Figure 5 Wireline Production Logging Toolstring

ISS-85 well (Nukhul Reservoir)
This well had been completed as an open hole cold oil producer in the Nukhul formation. However, the well soon started producing with 98% water cut at a total fluid rate of 6000 BFPD. Then the water production increased, dropping the oil production down to 20 BPD.

Two flowing surveys, (ESP at 70HZ and at 80HZ) and one shut-in survey of production logging were carried out. It was noticed that the productivity of the open hole interval changed with the change in speed of the pump.

Based upon the measured bottom hole pressures during the 3 surveys and the interpreted production rates, 4 reservoir zones could be identified in the open hole interval of 140 feet.
It was found that only about 30 feet of the open hole interval was contributing a little oil production at high drawdown. The remaining zones were contributing most of the water production. Two intervals which were the main source of water production could be identified. These flowing survey results are shown in Figures 9a and 9b.
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 Figures 9c.

The calculated flow profiles from the PL surveys matched well with the flowrates measured at surface, indicating that there was good confidence in the survey results.
A Selective Inflow Profile was drawn, which could identify the individual layer pressures and PI (Figure 10).

Based upon the above results a decision was taken to install and cement a 5’’ liner in the openhole interval. The water contributing intervals were isolated and the oil contributing intervals were then selectively perforated. This resulted in increasing the oil rate from negligible to about 70 BOPD as shown in Figure 11.

Figure-9a: ISS-85 flowing survey at 70 HZ

Figure-9a: ISS-85 flowing survey at 70 HZ

CSS-280 (Lower Dolomite reservoir)
This well had been completed as an open hole producer in the Lower Dolomite formation but the well did not produce any oil at all, although while drilling there had been good indications of oil presence in the formation, as per the logs.

After performing production logging in flowing and shutin conditions the zonal contribution intervals were identified, as shown in Figure 12a. It was seen that all intervals with resistivity less than 30 ohm-m had produced water,  whereas the oil pay interval had not participated in the production at all, even though the pump speed had been increased to create a higher drawdown, as seen in Figure 12b. Hence 30 ohm-m was confirmed to be the resistivity cutoff value for oil pay zones.

This confirmed that the water layers played a dominant role in the production from this well. The results of this log helped to redefine the resistivity cutoff for the pay zones in this field.

Figure 6 Operation of the electrical probe (FlowView)

Figure 6 Operation of the electrical probe (FlowView)

ISS-94 (Nukhul Reservoir)
This well had been completed as an open hole cold oil producer in the Nukhul formation but the well soon started producing with 6000 BFPD at 100% water cut. Another interesting observation was that closing in this well resulted in an increase in watercut in the surrounding wells. The surrounding water cut would go down again when this well was put back on production.

Two flowing surveys (ESP at 70HZ and at 75HZ) and one shut-in survey of production logging were carried out. 4 reservoir zones could be clearly identified in the openhole interval, as seen in Figure 13a. It was noticed that the productivity of the open hole interval changed with the change in speed of the pump.

A clear downward crossflow could be seen from the upper to the lower intervals during the shutin passes, as seen in Figure 13b. This explained the reason for the increase in water cut in surrounding wells during shut in.

PL results confirmed that the whole openhole interval produced water. Accordingly, the Nukhul formation was plugged back using cement to eliminate the cross flow, thus reducing the water production in the surrounding wells.  This well was recompleted to produce from the Gharandal formation.

Figure 6 Operation of the electrical probe Figure -7 Saddle bypass

Figure -7 Saddle bypass

The new Y-tool (Figure 7) along with ESP 400 series gave the ability to produce the wells up to 4000 BFPD. This is the first time such a y-tool was used and it achieved the same rates as the actually designed well production rates with this pump.

The pressure gauge measurements and SIP calculation confirmed the multi layer pressures within the same formation (Figure 9).
The production log results had a significant effect on the Issaran field development plan. Detecting the different layer pressures through the open hole section, identified the zones which contributed to increasing the water production from surrounding wells. Layers reading up to 30 ohm-m were identified as water bearing. This led to an increase in the resistivity cutoff for hydrocarbon production. Recompletion of these wells was done, by shutting off these lower resistivity zones, which led to a reduction in water cut from over 90% to less than 50%.

Hydrocarbon producing zones were thus better identified and Scimitar decided to implement a new development plan for the Upper and Lower Dolomite formations as well as the Nukhul formation. The plan is as follows:

A) Upper and Lower Dolomite formation

1) New Drilling wells
It was decided to complete some newly drilled wells as cased hole producers. This would be a pilot study, to compare the trade-off between shutting off the water production by casing the wells and the effect of the additional pressure drop through the perforated completion upon the productivity of the wells. Accordingly, new cased wells locations have been selected to test the whole field area (Figure 14). A comparison between the open and cased hole completion results is illustrated in Figures 15a and b. In both figures, the upper graph is for a cased producer, while the lower graph shows the case of an open hole producer. A good oil production (green curve) is seen from the cased wells (upper graphs) while hardly any oil production (green curve) is seen from the open hole wells (lower graphs) in both examples.

2) Current existing open hole wells
Select different injector well locations all over the field, install 5’’ liner and then selectively perforate the high oil saturation zones for steam injection into these zones (Figure 16). Care will be taken to avoid steam injection into highly fractured zones. This will force all the steam injection in to the oil zones which will reduce the heat loss to the water zones and maximize the heat transfer to the oil zones and pressure support.

The performance of the new liner wells before and after steam cycle has been illustrated in Figure 17 where wells CSS-86 (Upper Dolomite) and well CSS-121 (Upper Dolomite) have been chosen to test the idea in upper and lower formation. The figure shows that after installing the liner and selectively perforating the oil interval the incremental oil production is about 40 BOPD from each well.

B) Nukhul Formation
Increasing the drawdown in the Nukhul formation is the key factor to increase the oil production. Therefore, producing some Nukhul wells which have high productivity index and high reservoir pressure with the new ESP gave the ability to increase the production up to 20000 BFPD. This also had the favorable side effect of decreasing the water cut in some of the surrounding wells (Figure 18).

Figure -8 New modified Y-tool

Figure -8 New modified Y-tool

1. The Issaran field, being a complex reservoir, requires more than one monitoring tool for effective production surveillance and to perform optimization processes.

2. Using the modified Y-tool and new technology PL string, helped optimize the field production by identifying and shutting off the super-K layers that led to early water and steam breakthrough.

3. Reservoir management through an optimal field development plan is the key to increase production and consequently the recoverable reserves.

Mohamed. Samir, Wael Hassan, Maher Omara (Scimitar), Enas Thabet, Yousra Abugreen, Sameer Joshi (Schlumberger)


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