Many fields currently being drilled by the petroleum industry require the use of high angle, extended reach wells to access remote hydrocarbon deposits. Although pre-drill geomechanical modeling efforts are often carried out to define the mud weight window, many wells still experience significantly elevated drilling costs due to non-productive time (NPT) associated with wellbore instability. This can be attributed to many factors but is predominantly due to the lack of appropriate data while drilling. Since rock properties, stresses and pore pressure often vary from the pre-drill model predictions, critical wells frequently require real-time updating of the geomechanical models using relevant logging while drilling (LWD) data to facilitate accurate, real-time decision-making. In this paper, a case history is presented where memory-quality; high-definition LWD image logs were obtained via high-speed telemetry systems and used to assess wellbore conditions in real time. The high-resolution LWD images were used to detect faults/fractures, breakout and general hole enlargements. Relog images over known unstable sections were also obtained to ascertain the extent of time-dependent wellbore failure. These logs were successfully used to delineate the failure zones and apply the appropriate mitigating actions in real time. Introduction In today's petroleum industry, pre-drill geomechanical modeling efforts are routinely undertaken in fields where significant wellbore instabilities are known to exist. In many cases, comprehensive and robust pre-drill models can be developed that provide great value in that they can be used to design a well and drilling fluid program to mitigate wellbore instability (Evans etal. 2003). However, it is not always possible to construct a robust pre-drill model due to a variety of reasons (e.g., insufficient useful data). This combined with the fact that significant geological uncertainties may still exist, may limit the effectiveness of pre-drill geomechanical models when applied to a current drilling campaign. This fact is borne out by a Gulf of Mexico study (Dodson et al. 2004) suggesting that wellbore integrity-related incidents (hole collapse, kicks and lost circulation) represent a major source of non-productive rig time (despite the development of sophisticated geomechanical modeling capabilities). The study indicated that 37% of the total drilling NPT stems from geomechanical and pressure-related downtime, which consume 24 to 27% of the total drilling costs. Standifird & Keaney (2004) suggested that as much as 40–50% of all NPT is attributable to pore pressure, fracturing and hole instability. This costs the industry an estimated US$26 billion annually (Sweatman 2006). It is our contention that to effectively reduce or even eliminate this NPT, diagnosing wellbore failure in real time (and applying the appropriate mitigating action while drilling) is the key. This is most easily accomplished using high definition LWD image logs. Discussion of Image Logs LWD borehole images provide critically useful information in terms of borehole quality and position within the reservoir (Janwadkar et al. 2007; Lindsay et al. 2006; Lofts et al. 2005; Morris et al. 2006a, 2006b; Onu et al. 2008; Ritter et al. 2004; Stamm et al. 2007). When used in real time, these images can help with making decisions on drilling hazard mitigation and well placement during drilling (Lindsay et al. 2007; Morris et al. 2008). Recent advances in telemetry rates (Hernandez et al. 2008) show that higher resolution image quality, approaching or equaling that of memory data, is now available in real time. This technology has enabled the visualization of geomechanical features at sufficient resolution to be useful for real-time decision-making applications.

No abstract

Recent advances in high-resolution imaging are well perceived by the oilfield industry as an aid to refine reservoir characterization. The availability of high resolution image data in real-time allows for improved decision making and more optimal wellbore placement when navigating through the reservoir. However, the transmission of high resolution image data to surface is limited by the bandwidth offered by mud pulse telemetry systems. This paper describes a new concept for transmitting imaging data to surface in real-time. The concept includes a highly flexible and effective compression scheme that can be used for all currently available and future telemetry systems. Image transmission parameters can be modified via downlink commands at any time without interruption of the drilling process. In this way, image parameters can be optimally adjusted to suit the available telemetry rates and proportion of the bandwith dedicated to image transmission. This flexibility allows customization of the real-time imaging service to optimize the geosteering process and the ability to provide high resolution data over specific zones of interest. The relationship of pixel size, telemetry rate and the real-time image parameters of latency and redundancy are discussed with reference to real-time resistivity images from field measurements transmitted at different telemetry rates and the corresponding memory images. Introduction Wireline, featuring a multi-phase electrical communication line, provides a real-time connection between the downhole sensors and the surface system. Despite the immediate availability of data at surface, measurements are made long after the formation has been drilled. As a consequence, all major oilfield service companies invested heavily in Logging-While-Drilling (LWD) technology, aiming to measure formation parameters shortly after drilling with the rock in as close to pristine condition as possible. The immediacy of the measurement combined with the availability of downhole data at surface in real-time revolutionized wellbore placement. While bulk measurements still represent the vast majority of LWD logging data, a variety of oriented geophysical measurements such as gamma, density, caliper and resistivity data are available, allowing refined structural reservoir analysis. The visualization of such data via image plots simplifies interpretation. The amount of information available increases with image resolution, which is limited by the intrinsic resolution of the geophysical sensor. High resolution resistivity sensors (e.g. [Ritter et al., 2005]) are providing the highest image definition in industry with an effective pixel size of ¼″ × ¼″ in memory. Such highly resolved images are a valuable aid for a wide variety of applications including:–structural & fracture system analysis–thin-bed analysis to determine net pay thickness–sedimentary feature analysis for input to a depositional environment model–core depth calibration, core orientation and an alternative to conventional coring over long intervals. However, the exploitation of LWD data is currently limited by the slow communication through the mud column. The introduction of azimuthal geophysical measurements with the associated exponential increase in data potentially available further increases the gap between achievable and required data rates. While the need for additional downhole memory space can be resolved by introducing more memory space, recent increases in telemetry speeds can not keep up with the required channel bandwidths. For example, a high resolution azimuthal logging services can provide more than one hundred sectored measurements around the borehole. In memory each of these values is represented by 8 bits. A 500 ms acquisition cycle results in at least 2 kbps required to transmit such uncompressed image data sets to surface. Even sophisticated telemetry systems using the drilling mud as the communication medium allow communication speeds of only a few bits per second. Thus, data reduction and compression techniques (e.g. [Li et al., 2001], [Li and Wang, 2005]) are required suited to decrease the amount of downhole data by the magnitude of 10[3].

The Grane field causes significant challenges with respect to reservoir drainage and wellbore placement. The true resistivity profiles from offset wells are reflecting an irregular oil water transition zone in the field, likely to be caused by subtle facies variations and/or local variations in the oil water contact (OWC). In addition, horizontal wells penetrated many shales.Baker Hughes INTEQ has in collaboration with Hydro developed an extra deep resistivity service called DeepTrak™ to navigate at a distance of up to 12 meters from a resistivity contrast boundary. This service has been used in several wells.The tools as well as the surface software components provided full service capability and were used successfully to geosteer along the OWC at the required distance. In addition, the DeepTrak service was capable of detecting shale injectites, thus giving valuable insight for reservoir characterization and geological model update.By use of field data, it is demonstrated how the DeepTrak service can be used for accurate wellbore placement.

No abstract