Often in a mature oil field the question is asked: where is the remaining oil in place, ROIP, and how much oil is left? However, recently the reservoir and production engineers have added the third question to the quest for additional oil recovery. The third question deals with the uncertainty in the distribution and volume of the remaining oil in place in an under-developed part of the field. The integrated work of geologists, production engineers and computer geoscientists can lead to developing a volumetric model which utilizes formation properties, stratigraphy, and fluid surveillance data to determine the ROIP volume, as well as the associated uncertainty with regard to the ROIP. This presentation gives an overall view of the process which produces reservoir grids of original and current fluid in place volumes, and uncertainty factors to be utilized by reservoir and production engineers in targeting future development in the field. Also, these products can be used to enhance and optimize fluid surveillance exercises in the under-developed regions of the field.

The reservoir engineers have difficult time utilizing the popular tools such as reservoir simulators to answer the above questions. One of the reasons for this is the fact that most reservoirs are enriched with geological complexity that effect fluid movement, not easily simulated in a numerical model. Another complexity arises from the multiple drive mechanisms currently interfacing with each other in a large reservoir, making the setting of the boundary conditions a handsome task. For the reasons mentioned above, an alternative approach is taken that utilizes the existing field data to predict the ROIP, with a quantified uncertainty. This approach is based on time-lapse volumetric model, which uses the fluid surveillance data gathered in the field by various means. The fluid surveillance data can be interpreted from open hole logs, cased hole logs, production history, and even interpretation of the regional production trends.

The production engineers, with the help of geologists can utilize the existing open and cased hole logs, production history, or other types of surveillance tools in wells, to determine the vertical distribution of the fluid in those wells. Depending on the type of surveillance tools, the degree of confidence varies from well to well. Even with in a single well, the degree of confidence changes from one depth to another, depending on the surveillance tool that was used to identify the fluid type in the formation. For wells with no surveillance logs or data, an interpretation is made by development geologists who relies on the offset wells data. Obviously, the degree of confidence with this type of interpretation is lower than if the well had recent surveillance logs that showed its recent fluid distribution. After the geologists and engineers have interpreted all the wells in the field, and posted uncertainty ranks to each interpretation, a field wide exercise is going to take place that utilizes all this information.

A field-wide comprehensive fluid thicknesses grids and maps are generated that incorporate the structural complexity of the field by honoring the faults that have significant vertical throws. The reason generating grids with significant fault cuts is due to the field-wide understanding that majority of the faults with high vertical throws in the field are sealing faults and can act as flow barriers. The fluid thickness grids are calculated for both water and gas. The uncertainty indicator is also generated for each fluid grid. As a result, the entire reservoir is presented with small girds that contain net sand thickness for the three fluid phases, as well as three uncertainty indicators for each fluid phase. The end result will be the volumetric calculations based on these grids and generating an uncertainty indicator associated with it.

There are many applications that the above procedure can be utilized in, such as;

  • identification of the ROIP in underdeveloped regions of the reservoir and the degree of confidence associated with the economic success in such venture.

  • Identification of oil-rich regions that need additional surveillance work to increase the confidence in the ROIP, hence narrowing the uncertainty in the economic feasibility of the future development projects. Various fluid column maps can also be generated that will help geologists and engineer to identify the future drilling targets in an under-developed area of the field.

  1. The surveillance data, such as open and cased hole logs, production history and flow meter data can be utilized in a volumetric model to identify the remaining oil in place.

  2. Structural control and uncertainty can be incorporated into the volumetric model for quantifying risk associated with the reserves.

  3. The applications of this method can be found in optimization of fluid surveillance exercise and defining the degree of confidence with the remaining oil in place.

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