The compensated dual resistivity (CDRSM) tool is an electromagnetic propagation tool for measurement while drilling. The CDR tool provides two resistivity measurements with several novel features that are verified with theoretical modeling, test-tank experiments, and log examples. IntroductionThe CDR tool 1 is a 2 ~ 106-cycles/sec electromagnetic propagation tool 2 . 3 built into a drill collar. This drill collar is fully selfcontained and has rugged sensors and electronics. The CDR tool is borehole-compensated, requiring two transmitters and two receivers. The transmitters alternately broadcast electromagnetic waves, and the phase shifts and attenuations are measured between the receivers and averaged. Phase shift is transformed into a shallow measurement, R ps ' and attenuation is transformed into a deep measurement, Rad' The CDR tool has several new and important features. I. Rad and Rps provide two depths of investigation and are used to detect invasion while drilling. For example, in a I-a· m formation, the investigation diameters (50% response) are 30 in. for Rps and 50 in. for Rad' 2. ROil and Rps detect beds as thin as 6 in.; however, these measurements are affected differently by shoulder-bed resistivities and both require corrections in thin resistive beds. Rps has a better vertical response than Rad' Rad and Rps cross over at the horizontal bed boundaries; this crossover can be used to measure bed thickness.3. Both Rad and Rps are insensitive to hole size and mud resistivity in smooth boreholes. Borehole corrections are very small even for contrasts of 100: I between formation and mud resistivities. Rugose holes and salty muds together, however, can cause larger errors than indicated by the borehole-correction charts. In these conditions, borehole compensation is essential for an accurate measurement.An extensive research program was conducted to verif'y these features and to ensure that the CDR tool provides a high-quality log. To achieve wireline quality, the CDR tool's physics was studied thoroughly, and its environmental effects were modeled and experimentaly measured. Two theoretical models are used for the CDR tool. The first model treats the tool geometry in detail but assumes a homogeneous medium outside the tool. This model is verified by test-tank experiments and by air measurements. The second model assumes a simplified tool geometry but treats boreholes, caves, beds, and invasion in detail. This model is used to study environmental effects and to prepare correction charts. Experiments with artificial boreholes, caves, step-profile invasion, and horizontal bed boundaries verif'y the predictions of the second model. Finally, CDR logs are compared to wireline logs to demonstrate the new features.
Tight gas fracturing was pioneered in North America in the 1970's and 1980's, and also has a relatively long history in Germany. In the rest of the world, however, massive fracturing for production from tight gas formations (i.e. k < 0.1 mD) has been very rare, due mainly to poor economics, rather than lack of opportunities. A massive oil field was recently discovered in Rajasthan (northwest India). The field development would require significant amounts of natural gas for heating and processing of the waxy oil to be produced. The most economical solution to provide sufficient gas in this remote desert location was to produce it from a deeper formation discovered in the same area. The majority of the gas is contained in a volcanic section of basalts and felsics. A fracturing campaign was performed in 2006 on three deep gas wells to evaluate the post-stimulation production increase from a number of different horizons, with base formation permeabilities varying from 0.005 to 0.15 mD. A comprehensive program of core testing, fluids compatibility testing and pre-fracture diagnostic injections was performed. Fracture stimulation treatments were performed in three different sections of this very thick gas-bearing formation (> 400 m gross height). The formations ranged from the highest permeability (0.15 mD) Fatehgarh sandstones, to a lower permeability Felsic section (0.05 mD) and the lowest permeability volcanic rock (0.005 mD). All three types of rock were stimulated successfully and post-fracture well testing showed initial production rates agreeing with what was expected based on reservoir simulation. This important result supports the proposition that unconventional gas resources in Asian countries can be attractive when stimulation techniques perfected in other areas (i.e. North America) are applied 1. Introduction The Raageshwari Deep gas field was discovered by RJ-E-1 (Raageshwari-1) in 2003. It was the second well drilled on the Central Basin High (CBH), a 40km-long composite feature of elevated N-S-oriented fault terraces, arranged in echelon within the Southern Barmer Basin of Rajasthan (Figure 1). The Central Basin High (CBH) structure is divided into many major horst blocks, of which Raageshwari is the shallowest. Raageshwari Deep is a tight lean gas condensate field and is contained in an arrowhead-shaped horst block formed at the confluence of three fault trends and contains 4 reservoir bodies (Fatehgarh, Basalt, Felsic and Sub-Felsic).
Summary Most existing production of waxy oils occurs in high-permeability formations, and wax issues are mostly a problem in the production tubing or pipeline. Large reserves also exist in low-permeability formations that require hydraulic fracturing for economic production. Such a reservoir was recently discovered in Rajasthan, northwest India, overlying a more typical high-permeability formation, both with high-pour-point waxy oil. Because a very large amount of oil in place is present in the low-permeability formation, two fracturing campaigns were performed in two different horizons to assess the potential for successful well stimulation. The first fracturing campaign (on four wells) showed limited success. A study was performed to determine the reason for the failure, and significant changes were made for the second campaign. This paper documents the second campaign. This campaign featured the first successful use in India of heated fluids to stimulate a shallow, low-permeability, massive oil reservoir containing high-pour-point waxy oil. Introduction Numerous exploration and appraisal wells have been drilled in the RJ-ON-90/1 Block in Rajasthan, northwest India, targeting primarily the prolific Fategarh reservoir (Zittel et al. 2008). Many of these wells have encountered hydrocarbon-bearing-potential reservoir-quality rock in the shallower Barmer Hill formation. Fig. 1 shows the field locale, which is situated just east of the border with Pakistan. The Barmer Hill formation is a shallow (700 to 800 mTVDSS), rich to very-rich source rock and has been described generally as siltstone and claystone, with gas peaks and frequent oil shows. Because the source rock is lacustrine in origin, it has generated waxy oil. Fluid analysis from the Barmer Hill reservoir shows high pour points (>45°C), wax appearance temperature (WAT) several °C less than reservoir temperature (˜70°C), and in-situ viscosities of tens of centipoises. The in-place hydrocarbon may be large, especially in the north of the basin because of the Barmer Hill's high porosities (25-30%). Producing these hydrocarbons, however, is not easy. Natural-flow openhole tests in three separate wells in the northern fields showed mixed-to-poor results and very low permeability (1-10 md), as estimated from slug-flow-well test analysis. To prove the commercial potential of the Barmer Hill reservoir, a hydraulic-fracturing campaign was conducted.
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