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.

The Machar Field in the UK Central North Sea is a fractured Cretaceous chalk and Palaeocene sandstone oil reservoir, developed around a tall salt diapir. Machar was discovered in 1976 and, after a lengthy appraisal including extended flow tests starting in 1994, has been developed in a phased manner from 1998 through a multi-well subsea development. The steeper eastern flank has historically lacked coherent reflectivity on seismic data and has remained undrilled. The geological possibility of a reservoir on the east flank provided motivation for extensive seismic reprocessing between 2005 and 2007, and the seismic interpretation showed both a chalk and a sand presence in this area of the field. Simulation modelling suggested that a well here would deliver substantial incremental field volumes. Confidence in the new seismic interpretation reduced the subsurface risk associated with the area, and a new subsea drill-centre reduced the drilling risks and costs sufficiently to allow a Machar East well to be sanctioned. Successful well results in 2008 changed the entire perception of the field and acted as a springboard for further development including a sidetrack in the northern area and a third injection well to support the east, which was drilled and completed in the summer of 2010.

A variety of technologies have been used to address the challenges faced by Production Logging (PL) in high deviation and horizontal wells.Different configurations of array sensors have been deployed in these environments, to address well work objectives. There are situations where it is hard to differentiate between qualitative and quantitative answers. The question is how quantitative is high angle and horizontal PL data.Multiple datasets were studied and increased value can be added to array raw data by improvements to processing and interpretation. There is a need to differentiate between data acquisition and data processing and interpretation for high angle and horizontal PL. This paper describes a probabilistic approach developed to combine sensor responses into a quantitative solution. Individual sensors from a multiple array production suite can be used in combination with centralised (conventional) sensors to address reservoir conditions and well access challenges. Difficult well completion and rig height limitation increase the complexity level in such environments. This probabilistic approach is applied in a complex North Sea example to gain greater reservoir understanding with a rapid turnaround.

The newest generation of production logging tools consists of multiple sensors in multiple locations around the wellbore that incorporate 12 resistivity and capacitance probes and six spinners. The capacitance array tool (CAT™) determines the water, oil, and gas holdup in the wellbore. The resistivity array tool (RAT™) determines the holdup of hydrocarbons and water. Likewise, the spinner array tool (SAT™) consists of six bowspring mounted micro-spinners that enable the measurement of the velocity profile. These new tools provide a detailed examination of the flowing fluids in all types of wells, including highly deviated and horizontal wellbores, that is not available with the traditional center sample tools because of the wellbore conditions, especially with fluid segregation. With these 30 measurements, a system of quality control and processing was developed to enable both experienced and non-experienced engineers to determine whether or not the data was correct and valid. A quick analysis tool was developed to enable the field engineer and company representative to enter raw values from the two holdup devices and calibration values, and to determine the holdups from the two sensors. Similarly, entering the raw spinner counts, cable speed, and estimated spinner slopes into the quick analysis tool will provide an estimate of the velocity profile for the SAT spinners and the other spinners that are run. This quick analysis tool graphically shows the holdups and velocity in an easy-to-understand presentation for people who are not production logging (PL) experts. After the raw data in the field is validated, a complete analysis is provided. This analysis includes horizontal, vertical, and 3D images of holdup and velocity profiles; continuous displays of flow profiles; and a complete flow analysis consisting of the split of oil, gas, and water rates at both downhole and surface conditions. This PL data can be presented in standard log formats, spreadsheets, and other methods as needed. This process can be modified by either the service company or customer. Several examples are provided that show the capabilities of the new logging tools and the interpretation method used to determine the results. Introduction Phase segregation occurs in many wells, including those with little deviation from vertical; the lighter phases migrate to the high side of the wellbore, and the heavier phases migrate to the low side. In highly deviated and horizontal wellbores, traditional PL sensors, which are center sample tools or have single point measurements, may not provide the most accurate data as a result of the wellbore and well flowing conditions. These PL tools measure fluid properties, such as velocity, density, capacitance, temperature, and pressure. Tool position, or more accurately sensor position, may lead to incorrect interpretations regarding the flow environment of the well. New PL tools have been developed to help address the issues in deviated or horizontal wells. These new tools include two types of holdup measurements, capacitance and resistivity, as well as multiple velocity measurements. These new tools will be referred to as Production Array Logs (PAL) to distinguish them from the standard PL logs. These tools provide a relative bearing measurement that enables the location of each sensor to be determined. The velocity tool also includes an inclination measurement to aid in the analysis of the PAL data. The holdup tools have 12 measurement probes, and the velocity tool has six spinners. These tools, when run in conjunction with the standard tool string, provide multiple measurements around the entire wellbore. The interpretation of each tool individually is complex and, when combined with the other PAL measurements, the complexity increases dramatically. A new interpretation process was developed that combines the benefits of the newer sensors and addresses problems caused by the deviated and horizontal wellbores in the standard PL interpretation procedures.

Surveillance in deep water wells is cost prohibitive. There is a need for significant hydrocarbon production or water shutoff incentive to justify the intervention in such wells. The wells straddle multiple stacks of soft sediment reservoirs, being completed with open hole gravel pack. While laterally extensive barriers between various sands units might help the water shutoff / containment, the gravel pack annulus still provides a conduit for water to move upwards and jeopardize the shutoff success. In this campaign a meltable alloy was deployed to plug the flow in both annulus and screens. In deep water subsea wells, water conformance control is often attempted blindly without flow diagnostic surveillance or production logs as a minimum. This can impact the production due to plugging substantial hydrocarbon production or inadequate flow from the remaining zones. Candidate wells or techniques for shut-off require robust diagnostics to improve the success rate and limit loss of oil or gas production. In a recent well work campaign production logs were acquired to optimize the water shut-off. Well access is challenged by limited rigup height (short lubricator) and well deviation. The well trajectory impacts the phase presence, mixing and recirculation. It requires a short array of sensors conveyed on tractor. Logging while tractoring capabilities in surface readout mode is required to minimize the rig time, improve depth control and perform real time data quality assurance. The multiple mini-spinners, electrical and optical probes are all positioned to the well's vertical axis to capture all local changes in the flow regimes. Sensor arrangement is sufficiently compact in this tool to minimize flow disturbance by tool occupancy and movement along the well. Real-time profiling of the complex flow regimes during the acquisition provided better log control and understanding of the downhole phase dynamics. Changing the mindset about subsea deep-water reservoir surveillance paid dividends in water shutoff operations, both for immediate decision make and for longer term well and reservoir performance management. There was a net benefit by deploying a compact axial array production logging string that allowed accurate rate and phase allocation and further identification of zones to be isolated using an innovative plug-back method that significantly reduced the water production.