This paper describes a study undertaken to quantify the influence of mud-filtrate invasion on neutron compensated and density measurements acquired in vertical and horizontal wells. Our objective is to assess the influence of non-axial symmetric spatial distributions of fluid saturation on generic nuclear tools. The case of horizontal wells is of primary interest due to the complex spatial distribution of fluids around the borehole due to mud-filtrate invasion combined with fluid density contrasts, permeability anisotropy, fluid mobility, and gravity segregation, among other factors.
Invasion was simulated under the assumption of a water-base mud filtrate invading a gas-bearing formation. In addition, we simulated the mixing of salt between mud-filtrate and connate water. This approach accurately reproduced the effects of porosity, permeability, permeability anisotropy, relative permeability, capillary pressure, and fluid density, on the spatial distribution of fluids and salt concentration around the borehole.
The Monte Carlo N＿Particle (MCNP code was used to simulate the response of nuclear tools with consistent source-sensor configurations. To this end, we introduced generic models of thermal neutron and density tools referred to as Longhorn Nuclear Well Logging Tools.? Calibration of the simulations of nuclear measurements was performed against standard industry models to appraise their reliability and accuracy.
Results from this study indicate that presence of mud-filtrate invasion reduces, and even eliminates the effect of gas on neutron-tool measurements. It was also found that salt concentration of connate water caused an effect opposite to that of gas on the simulated neutron measurements. More importantly, the effect of salinity contrast, caused by fresh mud filtrate displacing salty connate water, increased the gas effect on neutron measurements for the case of shallow invasion. The effect of salt mixing was less critical for the case of density measurements. It was also found that presence of non-axial symmetric distributions of mud filtrate and salt concentration biased the estimates of density and apparent neutron porosity. Similarly, for the horizontal well case, depending on the location of the tool around the perimeter of the wellbore, both neutron and density measurements were influenced by non-axial symmetric spatial distributions of fluids resulting from invasion. This effect was most noticeable for tool locations at the top and bottom of the borehole. Tool standoff significantly biased both measurements.