The Magnolia Field in the deepwater northern Gulf of Mexico is a Plio-Pleistocene age mixed phase reservoir whose fluids are not in compositional equilibrium. Fluid heterogeneities have arisen principally due to (1) variations in maturity of the source rock from which the hydrocarbons were derived, (2) the extent to which biogenic methane has been incorporated into the fluids and (3) phase fractionation effects. These influences express themselves both in terms of bulk fluid properties such as gas/liquid ratio, API gravity and saturation pressure and minor compositional attributes such as hydrocarbon gas isotopic composition and gasoline range molecular ratios. Significant compositional variations that cannot be ascribed to gravitational fluid segregation occur within reservoirs that are demonstrably in pressure communication. These variations challenge the notion that hydrocarbon fluid mixing is geologically instantaneous and underscore the importance of testing assumptions regarding compositional equilibria in conjunction with reservoir studies. Although the state of disequilibrium impedes compartmentalization assessments at Magnolia, it provides both opportunities for fluid property and phase predictions and potentially a development setting in which geochemical surveillance techniques may be profitably employed.

Downhole nuclear magnetic resonance (NMR) measurements are evolving into a powerful formation evaluation tool, providing unique and critical information including formation porosity, pore-size distributions, bound-fluid volume (BFV), free-fluid volume (FFV), permeability, and fluid properties. Obtaining this information while drilling can have a significant impact on drilling and completion decisions. In addition, low rates of penetration common in many drilling environments can be advantageous in improving the NMR measurement statistics. This paper describes results gained during field tests of a new NMR logging-while-drilling (LWD) tool that has been designed to run in any standard measurement-while-drilling (MWD) bottomhole assembly. The new tool presents no special operational complications in terms of mechanical specifications or wellsite hardware and software, and it has been tested successfully in both real-time and recorded modes in a wide range of formations and drilling conditions. A key consideration in the design of this tool has been to deliver an NMR measurement of wireline quality with a minimum of interference to the drilling process. To this end, the tool is usable in various modes of operation (stabilized, unstabilized, while drilling, while reaming, etc.). The detrimental effects of tool motion on the NMR measurement are minimized through the hardware design. Motion-effects modeling and log examples address these issues. Having the capability of acquiring data in the conventional T2 mode, this tool offers a familiar interpretation strategy for those users accustomed to evaluating wireline NMR data along with a significant advantage in statistical precision over a T1 acquisition mode. Introduction Many oil exploration and production companies have been awaiting NMR technology capability in the LWD environment. With the inclusion of an LWD NMR tool in the bottomhole assembly (BHA) transmitting in real time, well-placement decisions that impact the overall economics and productivity of a well can be readily addressed. In highly deviated wells or in difficult logging conditions, an LWD NMR tool is the obvious replacement for wireline NMR data acquisition. Therefore, it is extremely important to design an LWD NMR tool that can produce industry-accepted NMR measurements and that can be added to the BHA with a minimum of additional time, cost, and disruption to the normal drilling process. Desirable features include precise and repeatable measurements, operation at high rates of penetration (ROP), measurements familiar to and accepted by the industry (similar to those made by wireline tools), high vertical resolution, flexibility of placement in the BHA, and real-time calculation and transmission of petrophysical and tool-motion information in the while-drilling, bit-on-bottom environment to reduce the need for additional passes after drilling. In the past few years, the LWD NMR tool described in this paper has been run in a large number of offshore wells on the shelf and in the deepwater of the Gulf of Mexico, offshore eastern Canada, the North Sea and Nigeria, and in a number of onshore wells in North America. During these field tests, two generations of the tool were run. The first-generation tool was powered only by batteries and a significant improvement came with the second-generation tool that was designed to address the large power consumption of T2-mode acquisition with the inclusion of a dedicated mud turbine. In conjunction with increasing on-board memory, the mud turbine power supply provided the second-generation tool with essentially unlimited downhole run time, which is a key benefit to any LWD tool when dealing with the highly variable nature of the overall drilling process and unscheduled rig operations. In a number of field tests with the battery-powered tools, data acquisition over target objectives could not be accomplished because the tool ran out of power before the objectives had been reached.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractLaminated formations create two major evaluation problems for petrophysicists. First is the classic low resistivity pay problem as seen in vertical wells. Layers of fine-grained sand, silt, and clay distributed within a hydrocarbon bearing sand will significantly reduce the apparent resistivity measured by an induction or propagation tool. The fine-grained layers hold high volumes of irreducible water, the sand will produce water-free oil or gas, yet the oil company may not even attempt to complete the zone. Second is the high angle well evaluation problem. The same laminated formation, when measured by an induction or propagation tool at moderate-tohigh relative dip, will exhibit an increase in apparent resistivity beyond that which was measured in the vertical well. Again, the accurate calculation of water saturation and hydrocarbon volume eludes the petrophysicist. Both of these classic problems can be solved with a common methodology that combines nuclear magnetic resonance (NMR) and resistivity anisotropy measurements.Case study wells are used to demonstrate two versions of this method. In the first version, horizontal resistivity (R h ) and vertical resistivity (R v ) are the initial inputs. With an assumption of the resistivity value of the silt-clay layers, we solve for the resistivity of the hydrocarbon bearing layers and the volume fraction of each layer. Water saturation (S w ) of each layer is calculated independently. Bulk volume hydrocarbon (BVH) of the entire formation is the sum of the products of the layer volume fractions and their respective BVH. The second version of the method uses the measurement of bound fluid volume from the NMR tool to determine the volume fraction of each layer. We then solve for the resistivities of both the silt-clay layers and the hydrocarbon bearing layers. The solution of BVH for the formation then proceeds as in the first method.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractLaminated formations create two major evaluation problems for petrophysicists. First is the classic low resistivity pay problem as seen in vertical wells. Layers of fine-grained sand, silt, and clay distributed within a hydrocarbon bearing sand will significantly reduce the apparent resistivity measured by an induction or propagation tool. The fine-grained layers hold high volumes of irreducible water, the sand will produce water-free oil or gas, yet the oil company may not even attempt to complete the zone. Second is the high angle well evaluation problem. The same laminated formation, when measured by an induction or propagation tool at moderate-tohigh relative dip, will exhibit an increase in apparent resistivity beyond that which was measured in the vertical well. Again, the accurate calculation of water saturation and hydrocarbon volume eludes the petrophysicist. Both of these classic problems can be solved with a common methodology that combines nuclear magnetic resonance (NMR) and resistivity anisotropy measurements.Case study wells are used to demonstrate two versions of this method. In the first version, horizontal resistivity (R h ) and vertical resistivity (R v ) are the initial inputs. With an assumption of the resistivity value of the silt-clay layers, we solve for the resistivity of the hydrocarbon bearing layers and the volume fraction of each layer. Water saturation (S w ) of each layer is calculated independently. Bulk volume hydrocarbon (BVH) of the entire formation is the sum of the products of the layer volume fractions and their respective BVH. The second version of the method uses the measurement of bound fluid volume from the NMR tool to determine the volume fraction of each layer. We then solve for the resistivities of both the silt-clay layers and the hydrocarbon bearing layers. The solution of BVH for the formation then proceeds as in the first method.

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