Third EAGE/SPE Workshop on Tar Mats

New insights to reservoir tar mats are made by integrating advances in asphaltene science and downhole fluid analysis (DFA). The Yen-Mullins model of asphaltenes has enabled development of the industry’s first predictive equation of state (EOS) for asphaltene concentration gradients, which has been referred to as the Flory-Huggins-Zuo (FHZ) EOS.

Steam or CO2 injection methods account for most of the oil recovered worldwide with Enhanced Oil Recovery (EOR) methods. Currently heavy oil production is less than 7% of the world's oil production; this percentage is not expected to increase dramatically without significant changes in reservoir management. Steam and CO2 have been used successfully since early 1960s -- steam in viscous heavy oils and CO2 mostly in pressurized light oil fields but also in some heavy oil fields. What limits a wider application is depth and high pressure for steam and CO2 availability for the relatively large inventory of light oil fields that exist worldwide. Although there is some overlap in fields that could benefit from either application, there are not many recorded attempts to implement both methods simultaneously. Air injection, although it was tried first as an EOR method, has not been widely implemented as in-situ combustion is difficult to control in shallow reservoirs and especially without water coinjection.

The drilling of exploration wells in thick Oil reservoirs has proven to be a major challenge for Oman Operator when it came to establish the presence of Oil in these wells. Traditional method consisted of using Nitrogen lifting to recover the oil, but this has proven to be very uneconomical option due to the extremely low pressure reservoirs. The option to recover the fluids using Beam pumps out of these low pressure/ high viscosity wells is possible, but as these wells are under exploration program, the client needs to assess the presence of Oil first before committing to bring in Beam pump/ PCP.

This paper presents an interpretation, evaluation and modeling a Tar Mat within an Upper Jurassic Reservoir, Offshore Abu Dhabi. The Tar Mat was observed in 6 cored wells out of 9 drilled wells in a Green Field.

To calculate reserves in heavy oil reservoirs, the amount of possible tar needs to be accurately estimated. This presentation discusses how Halliburton’s tar mapping technique was developed based on combining experience from different product lines over the years. The mapping technique combines successful NMR based tar identification techniques, heavy oil sampling, stimulation and chemostratigraphy.

This presentation is about the impact of a tar mat on steam development options for heavy oil fields with a strong bottom aquifer. In principle the standard steam development options, Cyclic Steam Stimulation (CSS), Steam Drive or Steam assisted Gravity Drainage (SAGD), can be used to develop heavy oil fields with strong aquifers. Efficient operation of such steam developments require that aquifer water influx is controlled and fields are operated at low pressure. A low reservoir pressure maximizes usage of the latent heat content of steam, which is the main source of heat transferred to the reservoir. Both effects, water control and low pressure, can be achieved by implementing an aquifer pump-off scheme. A potential additional benefit of an aquifer pump-of scheme is to use the produced water as source water for steam.