Successful Applications of Azimuthal Propagation Resistivity for Optimum Well Placement and Reservoir Characterization While Drilling

Abstract

Abstract We illustrate the use of a new technology for navigating and characterizing various types of oil reservoirs. Real-time images from Azimuthal Propagation Resistivity measurements provide a "map" of the resistivity patterns up to several meters around the wellbore. In addition, recently developed processing and quantitative interpretation techniques help guide the placement of the well and provide a new perspective of the formation. When navigating in gas drive reservoirs, the azimuthal resistivity measurement is used to maintain the wellbore at a prescribed distance above the oil-water contact. With its exponential sensitivity to distance, the measurement is able to detect even small changes in the distance to the oil-water interface. In a few instances, the azimuthal information provided by the real-time deep resistivity images indicates probable coning due to offset well production. Similar principles are applied in high angle drilling of water drive reservoirs. The deep azimuthal information allows the drilling engineer to maintain the wellbore at a prescribed distance immediately below a shale roof. The deep resistivity image from the azimuthal resistivity measurement also makes it easy to distinguish the roof from the occasional approaching shale lens. Whereas shallower reading LWD image logs (e.g. Gamma Ray and Density) only indicate a geological feature proximal to wellbore, the deep reading azimuthal resistivity measurement can provide geologic structure information at the reservoir scale. Visual displays show the subsurface surrounding the wellbore; quantitative algorithms accurately compute the distance, direction, and apparent dip for reservoir related geological events. A new conductivity unit named "Transverse Siemens" is proposed to help quantify the new azimuthal propagation measurement. Introduction The main objective of reservoir navigation is to stay for long intervals in the target zone, while keeping clear, at appropriate distances from boundaries including reservoir tops, oil-water contacts, shale lenses and other similar events. Typically wells need to be placed immediately below the roof, or a few feet above the oil water contact for the most effective sweep. In other instances, wells are driven to access multiple reservoirs or compartments. Proper well placement has helped to produce millions of barrels of "attic oil" or avoid costly early water breakthroughs.1,2 Successes in reservoir navigation have become more noticeable to the industry in recent years, as they enable higher hydrocarbon recovery from some well known, large oil fields. Steering decisions during reservoir navigation must be made quickly and accurately as the drill bit advances. Therefore, real-time information to the navigation engineer needs to be timely, as complete as possible. When traditional LWD propagation measurements predict an approaching boundary, they lack critical information as to whether that boundary is approaching from above, below or the side. Evasive actions are entirely different for each instance. Until recently, reservoir navigation has relied on deep reading propagation resistivity, without azimuthal sensitivity, to anticipate as early as possible the intersection with an approaching boundary. In many cases, at the time of intersection, an imaging instrument such as azimuthal gamma helps identify the direction of entry.3 This solution leaves an obvious time gap between the early detection of the approaching boundary and the late identification of its direction. Navigation experts help fill this gap through experience and local knowledge, but the risk of error can be significant. A new azimuthal propagation resistivity LWD combines the early detection of the approaching boundary with an early indication of its azimuth of approach.4 As previously disclosed, the information is presented as deep reading images. Through years of extensive use by geologists, downhole image logs have become familiar and easier to read.