Understanding of fluid movement in and near the wellbore is a crucial factor for effective reservoir management including successful remedial actions and field development planning. One of the key objectives in well surveys is to detect and locate sources of fluid flows behind multiple pipe barriers. The conventional Production Logging Tool (PLT) is run to detect fluid flow and identify the type of fluid under downhole conditions, but is limited to measurements only inside the wellbore. Similarly, other diagnostic techniques, such as cement bond logging, give insight only into the cement integrity and also have limited capabilities to detect cross flows behind casing.Recent developments in temperature and noise logging tools and advanced interpretation techniques have provided higher resolution and sensitivity, enabling the detection of previously undetectable leaks and fluid flow behind casing [1].In the present case, a water zone has been identified in a producing formation with High Precision Temperature (HPT) logging and Spectral Noise Logging (SNL) followed by advanced numerical temperature modelling using the TermoSim software application. SNL identifies flowing zones and differentiates between rock-matrix and fracture flows, and TermoSim then numerically models heat exchange between the wellbore fluid and the surrounding rocks and reservoirs. The resulting model quantifies fluid production from each reservoir unit. Conventional production logging (PLT) locates fluid entry points in the wellbore. The integrated HPT-SNL and PLT logging suite can trace the entire water path from the reservoir into the wellbore. This paper describes water source identification by an HPT-SNL-PLT logging suite deployed in several production wells of a Kuwait oil field. In some of the wells in this field, it has been found that water encroached into the perforations from a watered reservoir below through a channel behind the casing. In other wells, it has been found that cold water breakthrough occurred laterally from nearby water injectors. The exact identification of water sources is a crucial step in any further well remedial work to reduce or eliminate them from oil producing wells. [2]
Wellbore fluid flow profiles in both producers and injectors tend to change over time due to preferential depletion, formation damage, cross-flow, channelling or tubing or casing leaks. These changes can result in excess water production through channelling, coning, non-uniform water breakthrough (fingering) or out-of-zone injection – all leading to uneven flow, pressure and sweep profiles. Ignoring these complications can result in missing key points on reservoir behaviour, selecting wrong units for a 3D full-field flow model or misleading redevelopment planning. Therefore, it would be logical to check for changes in flow geometry before embarking on costly workovers, recompletion or infill drilling programs. This paper compares and integrates the results of conventional Production Logging Tool (PLT) surveys that use spinners and multiphase sensors with those acquired by reservoir-oriented production logging surveys employing a combination of Spectral Noise Logging (SNL) [1,2] and High Precision Temperature (HPT) Logging [3–5]. PLT and HPT-SNL produce similar results when wellbore and completion conditions are good but they may differ dramatically in cases of non-uniform formation damage, channelling behind pipe or plugging of perforations by scale. Generally, HPT-SNL would assess the flow geometry and invaded zones of the reservoir while PLT would point out where fluid enters or leaves the wellbore or tubing. The paper provides case studies from a mature offshore waterflooded field producing a mix of oil, gas, formation water and injection seawater, which complicates the identification of flow geometry and invasion zones and represents a challenge for reservoir engineers in developing proper drilling or workover programmes to target residual reserves [6, 7]. The HPT-SNL-PNL surveys and further studies described here led to successful workovers and drilling. The redevelopment results can be easily assessed by decline curve analysis. Introduction Since 2007, Dubai Petroleum Establishment (DPE) has performed more than 150 integrated PLT-HPT-SNL surveys to monitor vertical wellbore injection and production profiles that resulted in valuable and often surprising findings including unexpected water breakthrough intervals, bypassed oil zones and layers and water channelling behind casing in producers and injectors. These findings, in turn, led to a better understanding of how water propagated through reservoir from injectors to producers and were used to calibrate a 3D full-field flow model and identify optimum infill drilling locations for the redevelopment of the highly fractured crestal area of the field.
Gas or fluid ingress into the cement channel and then up to the surface through the surface casing annulus is called Surface Casing Vent Flow (SCVF), which causes Sustained Annulus Pressure (SAP) as a common occurrence in the petroleum industry. Gas may also migrate to the surface outside the outermost casing string, which is often referred to as external Gas Migration (GM) or seepage. In some countries with shallow coal reserves, gas migration sometimes occurs in association with coalbed gas (CBG) development. Dewatering the coal seams or lowered water levels in coal, whether induced by drought or by domestic aquifer pumping, can result in the release of methane and other natural gases in coal (NGC). Hydrocarbon gases released into the atmosphere is an environmental concern. More importantly, leaking fluids may contaminate subsurface fresh-water reservoirs, resulting in a major catastrophe for the environment and human population. According to the latest statistics, 6% of almost 270 000 operating and idle wells analysed in Alberta were found to contain leaks, 5.5% of them having SCVF and 0.5% gas migration [2]. Operators are bound by the Alberta Energy Regulator (AER) to identify and eliminate leaks and perform remedial operations as outlined in AER's rules and directives. Even if a well is to be abandoned, the operators must precisely identify the location of the leak and its source to perform a successful plug-and-abandonment (P&A) operation. P&A activities are non-revenue generating activities. The right diagnostic technology is critical for correct leak source identification to eliminate the costs associated with numerous unsuccessful attempts. The technique of Spectral Noise Logging (SNL) coupled with High Precision Temperature (HPT) Logging have extensively benefited oil industry outside Canada in accurately identifying fluid flow behind multiple casing pipe barriers and in locating leaks and their sources [3–5]. This paper describes two case histories for eight wells in the Western Canadian Sedimentary Basin (WCSB) in South Alberta region for two clients, where application of these techniques enabled gas leak source identification in a series of wells suffering from minute leak rates and also helped to discover some regional lateral flows and cross-flows.
One of the advantages of having a dual string completion is the option to develop and produce more than one reservoir simultaneously, whereas there are considerable disadvantages too and one of such is managing the integrity of the two strings intact throughout the life time of the well. In this paper, a case study from Ahmadi Field is presented highlighting the need for effective surveillance methods to identify issues with dual string completion and successful implementation of workover. Spectral Leak Detection (SPEC-LD) along with High Precision Temperature (HPT) tool was used to pin point the exact leak locations in the tubing strings prior to attempting the workover. Spectral Noise Tool is a passive tool with a high sensitivity hydrophone designed to record sound in the frequency range of 8 HZ to 60 KHZ. Analysis of noises recorded in various frequency ranges allows the location of flow-related features, such as wellbore flows, casing and tubing leaks, perforations, cement channelling and flows through the rock matrix and fractures. The SPEC-LD combined with a high-precision temperature (HPT) tool was found highly effective in pinpointing leak sources and quantifying it. Ahmadi Field is one of the prolific oil fields in South Kuwait and has many dual string completions. In this paper, a case study is presented where the SPEC-LD tool was effectively deployed to identify leaks in a dual string completion. The leak and communication between the strings resulted in higher water cut from the short string which was producing at zero water cut. Based on the logging results, the well was successfully worked over in a cost-effective manner to improve the production. The workover also resulted in reducing the water production from the well, thus saving water treatment cost prior to disposal. Knowledge of the tubing condition prior to deploying the workover rig was found useful and effective in selecting the proper workover procedure, tools and techniques. Blindly doing workover without knowing the tubing condition and well integrity generally results in the application of wrong workover processes and techniques causing overspending and safety concerns. Evaluating the condition of the tubing string and pinpointing the leaks was found useful in planning proper workover and thus minimizing the cost as well as bringing the wells back on line on minimum time. The main message from this paper is the benefit of using appropriate evaluation techniques prior to attempting any workover to save both time and money.
Over the last decade, an industry-wide shift to shale plays has occurred due to advances in technology allowing for the economic recovery of previously unattractive reserves. The primary objective of well completions in shale reservoirs is to increase the effective surface area and thus maximise reservoir contact, as the fracture network area is the most important factor affecting production from such reservoirs. Understanding which hydraulic fractures actively contribute to production in shale reservoirs is essential for well performance evaluation and improving current completion design. This last point is, in turn, essential for expected high productivities to offset high drilling and completion costs of unconventional oil wells. Today, unconventional plays are economically developed through stimulation of horizontal wells by hydraulic fracturing. Conventional production logging (spinner, borehole fluid salinity, density and flowing temperature surveys) often fail to identify which stage produces the greatest effect on hydrocarbon production. The integration of High Precision Temperature (HPT) Logging and Spectral Noise Logging (SNL) data is the most effective method of Reservoir Flow Analysis due to the large radius of investigation of these tools. For this reason, HPT-SNL can be effectively used for post-hydrofracturing diagnostics. More specifically, it can assess individual fracture contributions to the overall well flow. Spectral Noise Logging can not only pick up flow in rocks behind multiple barriers of casing and cement but also tell whether such flow comes from fractures, the rock matrix, perforations or completion components. High Precision Temperature (HPT) logging detects minute variations in fluid temperature caused by heat exchange that depends on the thermodynamic properties (heat flux, thermal conductivities, etc.) of completion components, formation rocks and fluids. Both technologies have been successfully applied in the shale-oil wells of the Permian basin and clearly differentiated between contributing and inactive fractures. Such information is critical for improving hydraulic fracturing job designs for wells to be drilled in the Permian Basin.
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