This paper reviews various inversion techniques applied to resistivity inverse problems, and proposes a fairly different approach. The advantages and disadvantages of various resistivity inversion methods are briefly summarized. A good inversion method must simultaneously minimize the effects of data error and model parameter errors. The necessary requirements for inversion of any geophysical data are a fast forward algorithm for calculating theoretical data for initial model parameters, and a technique for calculating derivatives of the data with respect to the model parameters. Unfortunately. the second requirement is not readily available for two-dimensional (ZD) or three-dimensional (3D) inverse resistivity problems. A few workers have examined the problem of resistivity inversion, but no claim has yet been made for an optimum inverse method for ZD or)D resistivity data. Most resistivity inversion methods are also restricted to surface electrode arrays. A multi-dimensional resistivity inversion method using perturbation analysis and reciprocity is proposed. The theoretical basis for this method is briefly developed. This approach is an efficient way of calculating the derivatives of data with respect to the model parameters because the inverse formulation thereby developed is a linearized form of the general non-linear problem. An attempt is also made to provide an update on the state of knowledge of resistivity inverse problems.

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Laboratory tests initiated in January 1997 demonstrated clearly that periodic, large-amplitude, low-frequency strain excitation of porous media leads to large flow enhancements. Based on these results, a new liquid flow enhancement technology for reservoirs was formulated, and a successful full-scale field experiment was executed in early 1999. Other field projects in 1999 through 2001 waterfloods in heavy oil cold production wells with sand influx confirmed the expectation that pressure pulsing, properly executed, increases oil production rate at low cost.The first trial showed that periodic application of large amplitude, liquid-phase pressure pulses increased oil production rates, decreased water-oil ratio, and increased the percentage of sand produced, even without large-scale injection. Though experience to date is in heavy oil, the process is general and will work in all porous media that have interconnected pore space. Furthermore, the method works in single-phase and two-phase liquid saturated cases, although the presence of large amounts of free gas is detrimental.Based on the field and laboratory work, and considering the nature of the physical processes, it appears likely that pressure pulsing will also help reduce coning and viscous fingering instabilities, help overcome capillary blockages, and result in more total oil recovery over time.

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