SPWLA 46 th Annual Logging Symposium, June 26-29, 2005 MONTE CARLO MODELING OF NUCLEAR MEASUREMENTS IN VERTICAL AND HORIZONTAL WELLS IN THE PRESENCE OF MUD- FILTRATE INVASION AND SALT MIXING Alberto Mendoza, The University of Texas at Austin, William Preeg, Consultant, Carlos Torres-Verdín, and Faruk O. Alpak, The University of Texas at Austin. Copyright 2005, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors. This paper was prepared for presentation at the SPWLA 46 th Annual Logging Symposium held in New Orleans, Louisiana, United States, June 26-29, 2005. 1 ABSTRACT 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. INTRODUCTION There is a strong industrial need to simulate the response of nuclear well logging tools in special borehole conditions and environments. Such modeling capability has not been widely available because specific details of the design of nuclear well-logging tools are needed to properly simulate their measurements. Service companies’ tool designs are usually proprietary and hence unavailable for accurate modeling by academic and oilfield technical experts. In this paper, we introduce a generic set of nuclear tools referred to as the “Longhorn nuclear well logging tools.” Such generic tools have been “designed” to match the response of service companies’ tools described in the open technical literature. The intent is to synthesize the most important properties and functions of standard measurement acquisition systems of nuclear tools, and to draw general conclusions on the nature of the measurements in complex rock formations. Results from our simulation work show that the Longhorn tools provide an acceptable match to the response of many of the service companies’ assumed borehole environmental properties. These