Late Pliocene Sapphire Sand Reservoirs Characterization Using Nuclear Magnetic Resonance: Sapphire-1 well, Case Study, WDDM, Egypt

Logging Technology and Acquisition

Nuclear Magnetic Resonance is a direct measurement of lithology independent fluid-filled porosity using non radioactive-source which is environmentally safe. The NMR petrophysical measurements and their applications have been explained by many authors [2, 3, 4]. The measured porosity doesn’t need any type of lithology correction but need fluid hydrogen index correction. The Magnetic Resonance delivers dependable estimates of moveable fluids in clastics and complex lithologies to optimize reservoir drainage strategies. A specific acquisition mode was used called dual wait-time mode which enabled NMR to identify water and hydrocarbon by using short-time 1 second and long-time 16 seconds wait times [5]. Magnetic Resonance has basic output and advanced applications (Figure 2). The standard basic results give total porosity which can be divided into clay bound water (CBW) porosity that represents the water in the internal structure of clay minerals. The amount of clay bound water reflects the volume of clay minerals in the reservoir [6]. The effective porosity is the result of the subtraction of CBW porosity from NMR total porosity. This effective porosity can be divided into two types, firstly the pores which contain free fluids and the pores which contain bound fluids which is bounded under capillary forces (Figure 3). The standard outputs also includes, permeability index and fluid properties (movable and immovable fluid) where fluid saturation is estimated using Dual Wait Time data for the movable fluid (water and oil) based on their intrinsic properties. Advanced applications of Magnetic Resonance include deliverables such as synthetic capillary pressures, relative permeability curves and grain/pore size distributions.

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Processing

The Nuclear Magnetic Resonance fluid saturation was computed using Dual Wait Time data which enabled to differentiate water and hydrocarbon based of short and lon wait-time as 1 second and 16 second respectively. Basic processing was conducted to generate key petrophysical parameters such as partial porosities with permeability index for different bins using 16times- stackings in order to yield good quality result. The calculation was done using proprietary computation with parameters such as gas gravity, density, longer, the pore size being coarse so each bin is representing a range of pore sizes starting from the smallest 0 m sec and ending by the coarsest 512 m sec [7]. In the present study_, T_2 is being used for advanced processing which gave result for estimation of the grain / pore size distribution. The computation for grain size was achieved from T2 distribution using pore geometric modelling. The basic concept of grain/pore size distribution analysis is shown in (Figure 4). The pore geometric model starts from the random dense packing of equal spheres. Adjustment for input parameters is done based on porosity reduction mechanisms primarily based on shaly sand mineralogy, since clay is high as evidenced by very high Gamma Ray response. In the processing of the random dense packing of equal spheres with input mineralogy, the pore scale modeling generates a pore network of tetrahedral pores and pore throats. At this step the T2 relaxation distributions from pore scale modeling were simulated with one single grain size at a time as imputed, as shown in (Figure 4), until a good match was obtained between measured and simulated T2 relaxations.

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Reservoir characterization

The porosity-permeability relationship using winland cross plot (Figure 5), showed that the Sapphire reservoirs in Sapphire-1 well could be classified into three main rock types. The first rock type (RT1), shaded with green color represents the best rock unit with a range of (0.1- 0.2 pu) porosity and the permeability rang (0.1 - 30 md). The second rock type (RT2) shaded with yellow color represents medium quality rock units having (0.05 – 0.40 pu) porosity rang with permeability rang of (<0.01 – 6 md). The lowest rock quality units (RT3) shaded with red color, it has porosity rang of (0.0 - 20pu) with permeability between (<0.01 and 1 md).

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Dynamic Rock Types Classification

The relationship between the flow and storage capacities was further analyzed using the slope trend lines (Table 1), by using the modified Lorenz cross plot (Figure 7), the graphical presentation of that relationship showed in (Figure 8). The slop trend line classified 24 sub units into three main dynamic rock units, the hid flow dynamic rock units, (i) the high flow dynamic rock units(DRT-1)with high steady slop, (ii) the moderate to low flow dynamic rock units (DRT-2) with moderate slope and (iii) the tightest dynamic rock units (DRT-3) with gentle to almost horizontal slope. The following is the detailed interpretation of the three different dynamic rock units:- i. High Flow Rock Unit (DRT-1): The (DRT-1) shaded with green color (Figure 7) included ten reservoir units with eleven steep slope trend lines, those rock units numbers are (12, 9, 22, 7, 11, 17, 14, 3, 16 and 21). This dynamic rock (DRT-1) unit considered the best rock unit, because it has around 19% of the whole storage capacity and 70% of the whole Sapphire-1 well flow capacity. ii. Medium Flow Rock Unit (DRT-2): This rock unit shaded with yellow (Figure 7), include nine gentle slope trend lines for eleven rock units (5, 10, (23&6), 4, 8, 15, 13, 18, 2 and 20). This dynamic rock unit included around 71% of the well storage capacity and 29% of the well flow capacity. iii. Low (Tight) Flow Rock Unit (DRT-3): This rock unit has three rock units (19, 1, 24) classified based on their three very gentle close to be flat slope (Figure 7). DRT- 3, has around 10% storage capacity and 1% flow capacity.

The integrated detailed NMR analysis along with Multimineral model based on conventional logging tool is shown in (Figure 9), this figure includes 16 tracks as the following:- 1) Reference Depth in MD 2) Gr and Caliper 3) Resistivity Curves 4) Density & Neutron. 5) Multimineral & Formation Fluid Saturation Model based on conventional logs. 6) Multimineral Model Net (Yellow Shading) and Net Pay Flags (Reg Shading). 7) T2 Amplitude distribution

8) Stratigraphical Zonation for Sapphire Reservoirs. 9) NMR zonation for Sapphire Reservoirs. 10) NMR Effective Porosity. 11) NMR Permeability. 12) NMR Bound Fluids. 13) NMR Free Fluids. 14) NMR Bin Porosity (Indicating both of grain size and pore throat). 15) NMR Movable Fluid (Yellow Shading) & NMR Non Movable Fluid (Brown Shading) 16) NMR Formation Fluid Characterization, CBW (Gray Shading), BVI (Blue Shading) and BVM (Yellow Shading).

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