A recently introduced azimuthal resistivity LWD imaging tool has been upgraded with advanced high resolution sensors that are capable of differentiating reservoir and borehole features down to a size of 0.4 in. when drilling in well consolidated formations. The high vertical and azimuthal resolution, along with 100% borehole coverage, yield an image quality comparable to that of wireline service for applications that include fracture characterization and formation evaluation. This paper describes a field test of the high resolution tool in 5 7/8" and 8 3/8" holes in Saudi Arabia and shows the application of LWD images for estimating carbonate reservoir producibility involving the characterization of secondary porosity. The LWD imager provides significant economic and logistic benefits, especially in slim horizontal sections; in addition, it can identify fractured zones with mud loss potential shortly after penetration. The real-time resistivity provides a good basis for accurate dip calculation and geosteering in general. In its default configuration, the high resolution tool is equipped with six high resolution sensors arranged in two rows. One of the benefits of the multi-sensor configuration, demonstrated by the field test, is the ability to validate the image quality by comparing data from various sensors. Another benefit is the depth correction achieved by correlating images from identical sensors located at various depths. The paper also discusses the fundamental principles behind high resolution resistivity imaging in conductive mud and makes extensive use of modeling techniques to characterize the sensor performance in various practical situations.

Within the Saudi Aramco organization, complex reservoir formation evaluation is regarded as a key discipline - fulfilling a critical role in maximizing production and facilitating accurate estimation of reserves. The timely acquisition of high quality and detailed petrophysical properties of the various reservoirs in Saudi Arabia is central to this role. Towards this end, Saudi Aramco Reservoir Description Division (RDD) introduced a new-generation multi-functional Logging While Drilling (LWD) technology into Saudi Arabia, for evaluation of on-shore complex carbonate as well as offshore clastic reservoirs. The traditional LWD measurements of propagation resistivity, gamma ray, density and neutron porosity along with advanced gamma ray capture spectroscopy measurements were compacted into a single collar. The electronic pulsed neutron source required for the spectroscopy measurements also provides a much improved neutron porosity measurement. This technology has been used in a number of Saudi Aramco horizontal wells in carbonate and clastic reservoirs. The data from these wells has been comprehensively evaluated. The results of the evaluation of the data acquired by the new tool and conclusions about the tool's introduction into Saudi Arabia are summarized in this paper. Introduction The geological complexity of the Khuff formation has made formation evaluation very challenging and new logging while drilling measurements are needed to properly evaluate the reservoir in real-time. Newly developed multifunctional logging while drilling tool has been run to date in four wells in Khuff reservoir and the data from these wells has been comprehensively evaluated. The new tool's exclusive capability to perform capture gamma ray spectroscopy and "Sigma" measurements is also examined. The incorporation of these advance measurements into petrophysical evaluations significantly improves the accuracy of mineral volume and saturation computations. Estimation of calcite, dolomite, anhydrite and heavy minerals fractions in the formation assisted in identifying better porosity development and producible reservoir intervals. In this paper examples of advance formation evaluation in Khuff-C reservoir are presented, these examples clearly show the significant contributions this new technology provides for the understanding of complex Khuff-C carbonate reservoir. Further to this, an example from offshore clastics; Wara member sandstone, Mouddud carbonate and Safaniya member sandstone is discussed for real-time petrophysical evaluation and detailed analysis for different clay minerals present. Geological Setting The Khuff formation is the earliest major carbonate sedimentation of the Ghawar field. The formation is deposited in late Permian during transgression of Tethys Sea over the Arabian Plate. Figure-1 shows generalized stratigraphic column.

Petrophysical workflows are primarily designed to process static data for traditional openhole logs (i.e., triple & quad combo) which can provide estimates of porosity, saturation, lithology and mineralogy. Dynamic data from core analysis can help to extend the log analysis for estimates of the rock’s dynamic properties such as permeability. However, there is normally a high degree of uncertainty in these estimates and formation testing and sampling (FTS) data is often required for reservoir condition calibration. The workflow for analyzing FTS data is highly specialized, and normally not performed by an operating petrophysicist, but by a specialist whose expertise covers FTS tools and applications, openhole logs, and reservoir dynamics. This paper bridges the gap between operational petrophysicists and FTS specialists by documenting standardized methods of FTS measurements and introducing an automated workflow for petrophysicists to conduct FTS jobs. This workflow begins with job planning, to data processing, decision making and recommendations. Unique characteristics and capabilities of this workflow are summarized below. The job planning methodologies are based on fundamental principles to determine how different tool technologies will perform in specific reservoir conditions. It will ensure that the most optimized selection of tools and modules are made for FTS operations. Measurement uncertainties and data qualification criteria are critical parts of the workflow. Expected uncertainties are compared to measured uncertainties to assess the results’ reliability. With test uncertainties, error bars are established for each measurement which are then incorporated into data processing such as gradient estimates. A quality grading algorithm is used to objectively provide a rating for each test based on the measurements. Exceptions for test anomalies are also handled and interpreted automatically. In real-time testing operations, automatic methods for objectively quantifying data quality and uncertainty are used for pressure, mobility, and fluid gradient analyses. Real-time decisions can then be made to either adjust pressure points or take fluid samples at critical locations to reduce uncertainty. Preliminary results of applying this workflow to automatically process test data and determine data quality more objectively, consistently, and efficiently are demonstrated using field examples.

Carbonate formations are highly heterogeneous with variations from grainstones to mudstones. Digenesis leads to changes of the rock's original nature, like dolomitization, vugs, layering and fracturing. These variations have effects on rock quality in terms of porosity and permeability. Similarly, clastics, including shaly sands, can be quite challenging in terms of accurate formation evaluation. Although extensive logs are run for the petrophysical evaluations in these formations, the use of advanced wireline formation testers (WFTs) greatly aids reservoir description. Standard formation evaluation tools and techniques sometimes result in low level of confidence in identifying and quantifying the presence of hydrocarbon in certain reservoirs. The application of modern wireline formation testers has become a useful tool in minimizing uncertainties in situations where we have low confidence in log evaluation. Some borehole conditions and reservoir architectures, like fractures, vugs, and low mobility and mud losses could also pose some challenges while performing formation testing. In this paper, several examples and case histories of the application of advanced wireline formation testers across varieties of rocks with fractures, vugs and different borehole conditions are presented. Results indicate that reservoir heterogeneities can be described and quantified more accurately with the integration of dynamic data to aid reservoir characterization. This paper also demonstrates how to handle the challenges of detecting the early traces of hydrocarbon arrival for real time decisions during pressure testing and sampling.