In this paper, we demonstrate the feasibility of jointly inverting 4D gravity data collected on the surface and in a borehole setting to characterize fluid contact movement over time. As technology rapidly advances towards a practical borehole gravity meter capable of collecting data in horizontal monitoring wells, the foundations for properly inverting and interpreting these valuable data must simultaneously be developed. We demonstrate that 4D gravity method may contribute to improved production efficiency and reservoir management in-between the more traditional, and expensive, 4D seismic surveys. In our presentation, simulations are performed using a representation of the published Jotun Field in the Norwegian North Sea, a well-studied site demonstrating successful application of time-lapse (4D) seismic method after early rise in water cut, and therefore decline in production. Results demonstrate that 4D gravity monitoring in-between seismic surveys may have predicted this early water-cut at this site, thus providing additional valuable information to better optimize reservoir management.
Gravity technology is continuously advancing towards smaller and cheaper sensors capable of monitoring subtle time varying density changes during production and water injection. In particular, recent hype (more appropriately referred to as rumor at this stage) even hints at the development of the next generation of borehole gravity sensors capable of collecting high quality data in the horizontal well environment. As these technologies continue to progress, the prospect of jointly utilizing surface and borehole gravity method as an additional tool for effective reservoir management becomes more viable. The approach should prove an effective, yet relatively cheap means of filling the void of knowledge between the more costly 4D seismic surveys.
To demonstrate the feasibility of the proposed method, we simulate density change from brine injection over a model created to mimic the Norwegian North Sea Jotun Field published by Gouveia et al., 2004. We utilize relevant parameters for the Jotun reservoir, such as geometry, thickness, depth and porosity. Geometry of the simulated reservoir is illustrated in Figure 1.
To demonstrate the need for such an inter-seismic monitoring approach, we simulate expansion of the Oil Water Contact (OWC) over time. During injection and production at Jotun in the early 2000’s, a continuous horizontal shale layer, originally believed discontinuous within the reservoir resulted in approximately half the predicted vertical movement through the reservoir. To compensate for the decreased vertical sweep, an increased rate in lateral movement of the oil water contact (OWC) arose, which led to early rise in water cut, declining production and eventual well infill (Gouveia et al, 2004). In the following, we simulate increased lateral movement of fluid contact due to shallow vertical sweep using a reservoir model simulating the Jotun field scenario. We show that the time change in density contrast from increased lateral movement of the OWC may be successfully recovered by jointly inverting surface and borehole gravity data during reservoir monitoring.
Density Model
The model we utilize for this study is a representation of the Norwegian North Sea Jotun Field. To construct a scenario representative of the true site, we built the model based on parameters representative of the field, extracted from the work of Gouveia et al., 2004.
