Inflow and outflow signatures in flowing wellbore electrical conductivity logs

Abstract

Flowing wellbore electrical-conductivity logging provides a means to determine hydrologic properties of fractures, fracture zones, or other permeable layers intersecting a borehole in saturated rock. The method involves analyzing the time-evolution of fluid electricalconductivity logs obtained while the well is being pumped and yields information on the location, hydraulic transmissivity, and salinity of permeable layers, as well as their initial (or ambient) pressure head. Earlier analysis methods were restricted to the case in which flows from the permeable layers or fractures were directed into the borehole. More recently, a numerical model for simulating flowing-conductivity logging was adapted to permit treatment of both inflow and outflow, including analysis of natural regional flow in the permeable layer. However, determining the fracture properties with the numerical model by optimizing the match to the conductivity logs is a laborious trial-and-error procedure. In this paper, we identify the signatures of various inflow and outflow features in the conductivity logs to expedite this procedure and to provide physical insight for the analysis of these logs. Generally, inflow points are found to produce a distinctive signature on the conductivity logs themselves, enabling the determination of location, inflow rate, and ion concentration in a straightforward manner.Identifying outflow locations and flow rates, on the other hand, can be done with a more complicated integral method. Running a set of several conductivity logs with different pumping rates (e.g., half and double the original pumping rate) provides further information on the nature of the feed points. In addition to enabling the estimation of flow parameters from conductivity logs, an understanding of the conductivity log signatures can aid in the design of follow-up logging activities.1

Multirate flowing fluid electric conductivity logging method

“…It has also been applied extensively by Pedler et al [1992], Evans et al [1992], and Bauer and LoCoco [1996] in shallower wells down to about 100 m. Additions to the analysis methods were made by Evans [1995]. More recently, Doughty and Tsang [2002] further improved the analysis method, on the one hand to allow analysis of natural regional flow, and, on the other, to provide distinctive signatures to help with log analysis.…”

Signatures in flowing fluid electric conductivity logs

“…All of these steps are carried out in conjunction with numerical simulations using BORE II, which is also used to optimize or refine the feed-point flow rates and salinities. For convenience in field applications, the above procedure has also been formulated in flow chart form, providing step-by-step guidelines for the analysis of flowing FEC logs (Doughty and Tsang, 2002).…”

“…The steady-state mixing rules (Eqs. (3)-(6)) must be modified when upflow is present (Doughty and Tsang, 2002), and in general it is not possible to determine all feed point properties individually.…”

“…(16), an outflow strength q out Z0.029 m 3 /hZ 0.49 L/min is obtained. An alternative procedure to determine q out , which involves correcting the M(t) integral to account for the fact that the limits of integration do not include entire concentration peaks Doughty and Tsang, 2002), provides similar outflow strength.…”