Three-Axis Borehole Gravity Logging for Reservoir Surveillance
Authors: Lofts Jeremy , Zett Adrian , Clifford Peter , Li Yaoguo , Krahenbuhl Richard , Seshia Ashwin 
Published: March 2019

Abstract

Objectives/Scope
The advantages of measuring gravity in the borehole environment have been well established in the literature and through first-generation instruments. These measurements can be very effective for directly imaging mass distributions at-depth in the subsurface and at large-distances from well bores. To date, a breakthrough has been limited by the sensor form factor (size) and measurement stabilization. Newly emerging MEMS three-axis microgravity technology, deployable by wireline, is showing the potential for a host of applications and capable of realizing the long-coveted advantages. For reservoir surveillance, a primary application is to perform more pro-active, frequent flood front monitoring. With its large volume of investigation, the proposed three-axis borehole gravity measurements would complement as well as fill the existing gap between traditional methods such as Pulsed Neutron and 4D seismic. Further applications extend to saturation monitoring, by-passed pay, and thin-bed identification.
In conjunction with a collaborative program to develop a three-axis gravity sensor that is now being incorporated into a 54-mm diameter wireline tool with a targeted sensitivity ≈5 μGal (microGal), we have carried out extensive numerical studies to understand the signal strength of such measurements produced by the dynamic processes in different types of reservoirs, and demonstrate the capabilities and limitations of borehole gravity and its potential use within a revised reservoir surveillance plan.

Methods, Procedures, Process
We show examples of forward modelling data from reservoirs with varying fluid displacement mechanisms. Reservoir porosity and saturation data are used to model the predicted three-component (i.e., vector) gravity anomaly (gz, gx, and gy) responses along the wellbore in a variety of wells as the fluid-water front progresses through the field and the modelling included both producing wells and injector wells. The paper will present a description of a forward modeling workflow, simulation studies based on real reservoir data and the validating measurements.

Results, Observations and Conclusions
The paper examines the results of the forward modelling and compares the results with the target sensitivity of the new three-axis borehole gravity sensor. The results will show that a wireline deployed three-axis gravity tool with a noise floor of ≈5 μGal will provide additional important surveillance to constrain reservoir models. It will also provide vital information to help reduce uncertainty when actively managing waterfront movement (sweep), secondary recovery and for detecting early breakthrough of water; and for monitoring and adjusting strategy when producing through reservoir depressurization. The described workflow is seen as very important for any future survey that planning to understand the time-lapse gravity signal and the feasibility of time-lapse gravity surveillance under different reservoir conditions.

Novel/Additive Information
A three-axis borehole gravity tool with a form factor enabling it to be deployed through cased hole and into deviated and horizontal wells is completely novel and has not been presented previously. A workflow that understands survey feasibility and optimal survey-time intervals is novel. A systematic and comparative study of three-axis borehole gravity responses through modelling of water flood in a set of reservoirs located on different continents is novel and has limited previous work.