Crosswell Electromagnetic Tomography in Haradh Field: Modeling to Measurements

Abstract

Crosswell electromagnetic (X-well EM) resistivity is emerging as an intriguing technology for reservoir surveillance. It provides a cross-sectional resistivity image between two wells and has the potential to provide fluid distribution at an inter-well scale. It can be used for identifying bypassed hydrocarbons, monitoring macroscopic sweep efficiency, planning infill drilling, and improving effectiveness of reservoir simulation. It can be deployed for one-time or time-lapse surveys. A crosswell EM technology trial project is being conducted in an Upper Jurassic carbonate reservoir, at the Ghawar Field in Saudi Arabia, to monitor the movement of injected water flood front and map the fluid distribution. The project site is in Ghawar's southern region, Haradh Field, and consists of three wells in the oil-water contact zone where peripheral injection water may have produced an uneven flood-front distribution. Significant drilling and well deepening were required prior to the deployment of tools in the three-well triangle. In fact, one new well was drilled and two other wells were deepened by more than 200 m, so that good volumetric coverage could be obtained at the oil- water contact zone. Extensive logs, core and formation tests were also acquired to provide deterministic saturation profiles at the near wellbore region. Formation evaluation in the project area indicates that one of the wells was fully swept while a second well, some 400 m away, was not. In July 2007, crosswell EM surveys were acquired across the three Haradh wells. In spite of the large well separations, the acquired EM data had good quality, and good stations repeatability. Preliminary processing has revealed a structure consistent with the background structure but a clear image of the oil-water contact is yet to be made. Introduction The Haradh Field is in the southernmost part of the greater Ghawar Field - the largest single oil field in the world (Figure-1). Arab-D is a 100-m thick, highly prolific, upper Jurassic reservoir comprising a carbonate sequence of grainstones, packstones, and wackestones 1. The original sedimentary textures have been altered in many places by leaching, recrystallization, cementation, dolomitization, and fracturing, which have caused a variety of pore types 2 to coexist in Arab-D. Flood-front movement can be uneven in some parts of the reservoir. Reservoir porosity ranges from less than 10% at the base to over 30% at the top while permeability ranges from few millidarcies to more than one Darcy. The Arab-D reservoir in Ghawar has historically been operated at relatively low depletion rates. Flank water injection is being carried out to maintain pressure and to improve sweep efficiency in this reservoir. With current inter-well spacing, about 1 km, determining fluid distribution behind the flood front is a key challenge to maximizing recovery from this reservoir. Traditional reservoir fluid monitoring techniques, e.g. pulsed- neutron logs (PNL) and resistivity logs, have investigation depths ranging from few inches, for PNL logs, to about 3 m for the deepest resistivity logs 3. Therefore, they cannot be used effectively for flood-front monitoring at the inter-well scale.

Synergizing Crosswell Seismic and Electromagnetic Techniques for Enhancing Reservoir Characterization

“…In practical applications of crosswell imaging, wells that are farther than 2 kilometers apart may not be accurately imaged and were not taken into account in both the 2D and 3D reservoirs (Marsala et al 2007;Schlumber Imaging Services Report 2010). This implies that the cross section between Injector I01 and I02 is not considered, whereas the cross section between Producer P09 and Injector I01 is.…”

“…Salt concentrations were assumed to be constant throughout the reservoir, as typically considered in the industry (Marsala et al 2007; Schlumberger Imaging Services Report 2009; Zhdanov 2010). Even though, for resistivity logging, the salt concentration close to the wellbore may considerably change when injecting water (Alpak et al 2006), the salt-concentrations differences within the reservoir may be rather insignificant for crosswell reservoir applications, or necessitate a more-comprehensive treatment, incorporating the chemical reactions the salt undergoes in the reservoir (Marsala et al 2008).…”

“…In contrast, EM data showed a strong dependence on the water-saturation changes, which is consistent with Archie's relation. EM data depend strongly on the water-saturation levels in the reservoir and therefore may more easily distinguish between heavily and less heavily waterflooded reservoirs (Marsala et al 2007(Marsala et al , 2011Lien 2013) that is rather challenging with seismic techniques. Furthermore, because of the strong salt-concentration dependence of the EM signal, EM techniques are further able to distinguish between water influx from an aquifer and the water that is injected from the surface (as outlined in the 3D case) because of the significant salt-concentration difference between the two fluids.…”

Enhanced characterization of reservoir hydrocarbon components using electromagnetic data attributes

“…This helps determining flow propagation paths and highly water saturated areas, as well as areas with significant remains of hydrocarbons. EM imaging techniques have successfully been employed by Marsala et al [11] in collaboration with Schlumberger, where Saudi Aramco performed field tests in the Haradh field for monitoring the movement of injected water flood front and map the water saturation [12]. The Haradh field is an upper Jurassic carbonate field and the test site consisted of three wells in an oil-water contact zone that may have encountered uneven flood-front distributions.…”