Effect of invasion on rockphysical and petrophysical properties

Project Details


This project aims at developing a method for interpretation of the sonic log signal, where invasion by drilling fluid causes anomalous log readings.

Sonic logs are used to measure acoustic velocities of formations penetrated during drilling. Together with data from other logging tools, such information is applicable for fluid identification and quantification in reservoirs. Sonic velocities also provide ties to seismic surveys (AVO, 4D analysis). An important tool in the interpretation of sonic log measurements is fluid substitution, which is a procedure developed by Gassmann (1951). Application of fluid substitution to multiple-phase saturated regions, such as the invaded region has been shown to be non-trivial, displaying hysteresis depending on formation wettability, formation fluids, and invading fluids (Cadoret et al., 1995). Hysteresis has also been found in electrical resistivity as quantified by the electrical saturation exponent (Van de Verg et al., 1999). These dual observations indicate correlation between hysteresis and homogeneity of fluid phase distribution.

Previous studies have addressed systems of mixed gas and water and have led to definition of theoretical bounds for the velocity response of two-phase saturated rocks (Cadoret et al., 1995). In preliminary studies to this project, we have however shown repetitive breaching of these bounds primarily in rocks with heterogeneous pore size distributions. Breaching the bounds shifts the influence of invasion beyond the limits of the present interpretation. This motivates further quantification of the effects of invasion on sonic logs (Sørensen, 2010).

Futhermore, sonic velocities and relative pore fluid related changes are frequency dependent. Sonic tools operate at much higher frequency than seismic surveys, while velocity measurements in the laboratory are conducted at even higher frequency.

Proposed work
This project comprises two parts. In the first part we will address the question of elastic and electrical properties of a two-phase saturated simple pore structure of quartzitic sandstone. Different saturations will be systematically studied according to a drainage and subsequent imbibition process. In the second part of the project similar procedures will be applied to greensand, in order to study the effect of a heterogeneous pore structure. All results will be discussed in context of rock-physics modeling. The results from laboratory data and rock-physical modeling will be used to formulate a strategy for improved interpretation of sonic logs in combination with other logs.
Effective start/end date15/08/201015/08/2013