Effect of Water-Soluble Gases on Production Decline, Production Stimulation, and Production Management
Authors: Hayatdavoudi A. 
Published: December 2007

With the help of industry and others, the author of this paper has compiled 16 years of field production and several pressure buildup data from two wells. These wells have been drilled in offshore Louisiana. The production and well test data cover both pre and post acidizing periods.

The first objective of this work was to investigate whether the monthly production data revealed any hidden precursor (signature) to the impending Formation Damage and drop in oil production. If successfully achieved, the precursor would obviously assist us in two ways. One such way is that it would help us understand the reasons for a premature drop in Oil Production or Formation Damage. The other is that it would give us adequate time to take the necessary, preventive remedial action(s) prior to the onset of a serious Damage or an unusual drop in oil and gas production.

The second objective was to introduce a simple technique, compatible with the present methods for interpreting the important features of the production and well test plots. The author, in this paper, has combined the Fourier analysis and the Action Parameter analysis for this purpose. These simple technique help us to manage the production strategy optimally as a part of the overall reservoir management package. Again, this can be accomplished in two ways. It could be done on a real time basis by automatically controlling the pressure and flow rate of all the wells in the reservoir(s). Another way involves a real time production optimization basis by installing modern, imbedded, on-site computers connected via an automatic control to a closed loop servo-controlled choke and separator system on an individual well. This well can easily be networked with other wells in the reservoir.

Interestingly, for the first objective, the Fourier periodigram coefficients, frequency, and phase changes of the gas-water ratio of the monthly production data indicate a certain predictable precursor (signature) to the drop in oil production and Formation Damage prior to the onset of the real production decline. In this work, for the sake of brevity, the author has only resorted to the Fourier Phase changes in the analysis of Oil Production data. Furthermore, it is found that the existence of the precursor may be attributable to some portion of the reservoir "drive" energy that is wasted either as a pressure drop through the well down hole or surface equipment or in the perforation and formation as a total skin (completion, turbulence, formation damage, etc). The source of this energy may be rooted in the difference between the solubilities of methane, CH4, carbon dioxide, CO2, and other gases in the reservoir water. The results of the Fourier analysis of the monthly production data could be used to control the worsening or remedying of the Formation Damage before the implementation of some low cost remedial measures that turn into a costly or ineffective stimulation (see post-stimulation Oil Production data in Well No. 1.), or premature workover. Furthermore, this type of analysis could provide the industry with an efficient means of real time production optimization.

For achieving the second objective, the author has used a simple Hamilton Integral in the form of an Action Parameter, As=pt2, or As= Et, (As = Action Parameter, p = power, E = energy, and t = time). When this integral is combined with Fourier analysis for the pressure build-up data, it reveals several clear results on the well test plots. The first result of this technique is the appearance of a sharp demarcation line between the transient, pseudosteady-state, and steady state times and their attributes like storage, boundary effect, seals, transition, onset of radial flow, dual porosity effects, and other aspects of well tests on the plots. The clarity of the results may be attributable to the sensitivity of the Action Parameter to time event. The many features of the new analysis technique may be used as a suitable substitute for a first or second "derivative" plot when the standard derivative plots remain too "rough and wavy" and ambiguous. The second result of this technique is the appearance of a clear and well-behaved slope of the steady state portion of the build-up plot which may be, again, a suitable substitute for when, for example, the steady state portion of the standard Horner plot cannot be used or interpreted clearly. It is expected that this technique, with proper calibration with the Scale (−10 to +200) for the skin caused by Formation Damage, and other scales for Storage, Productivity Ratio, Production Efficiency, Post-stimulation effects and Stimulation Efficiencies, and other parameters, to give much clearer results than what is available to the industry at this time.