Interpretation of Horizontal-Well Production Logs: Influence of Logging Tool
- E. Ozkan (Colorado School of Mines) | C. Sarica (U. of Tulsa) | M. Haci (Welltec Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- May 2002
- Document Type
- Journal Paper
- 84 - 90
- 2002. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 3 Production and Well Operations, 4.6 Natural Gas, 2 Well Completion, 1.8 Formation Damage, 3.3.1 Production Logging
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This paper investigates the disturbance of horizontal-wellbore flow conditions by a production-logging tool and its influence on wellbore flow-rate and pressure measurements. An analytical model is used in the investigation, and single-phase liquid flow is assumed. It is shown that the influence of the production-logging tool manifests itself in low-conductivity and high-rate wells; this effect may be ignored if the dimensionless well conductivity exceeds 500 and the Reynolds number is lower than 100,000. Results indicate that production logs in low-conductivity horizontal wells may not display low-permeability zones along the well.
Production logging has been an important tool to analyze the performance of horizontal wells in the last decade. In principal, there are two fundamental uses of the information obtained from production logging under single-phase flow conditions. Qualitatively, production logging provides information about the flow profile and low- and high-influx regions along the lateral extent of the well. This information is extremely valuable for well completion and stimulation decisions and also helps interpret the discrepancies between expected and observed well performances. Quantitatively, the measured flow rate and pressure profiles may be used to decouple the reservoir performance from the wellbore hydraulics when low horizontal-well conductivity and nonuniform skin distributions are of concern.1-3 Now increased well lengths and high production rates make horizontal wells more susceptible to finite conductivity, and although this is not a simple task, a meaningful evaluation of well performance relies upon our ability to remove the wellbore-related effects from the observed data. The accuracy of the current production-logging data, however, does not yet permit quantitative evaluations, and the use of the information obtained by production logging is essentially qualitative.
There are two major reasons for the errors associated with the data acquired by production-logging tools: the limitations of the tools presently available in the industry and the disturbance of the original flow conditions in and around the wellbore because of the production-logging tool and coiled tubing. The errors associated with the tool itself call for the development of more sensitive and accurate tools and measurement techniques. This task has already been undertaken by the industry, and substantial improvements may be expected in the near future. On the other hand, the fact that the tool used for the measurements disturbs the original flow conditions in the wellbore is a problem that is likely to survive despite the developments in production-logging tools. Recovering or inferring the original flow conditions in the wellbore from the production- logging data is an extremely complex problem, and a simple and tractable solution may not be always available. Therefore, the conditions under which the information obtained by production logging is meaningful should be of great practical interest.
The previous discussion constitutes the fundamental motivation for this paper. We use an analytical model to investigate the tool's influence on production-logging data. The errors caused by tool sensitivity or measurement techniques are outside the scope of this study. The model used here has been derived from the coupledwellbore- reservoir model described in Refs. 1 and 2. The analytical expression and the numerical results presented in this work provide insight about the parameters controlling the effect of the production-logging tool.
We first introduce the model used in this study, and our discussion continues with example cases to highlight the conditions under which the production-logging tool significantly affects measurements. Finally, we evaluate the results and provide general guidelines to determine whether production logging can yield meaningful information concerning the original flow conditions of a given horizontal well.
The analytical model used in this study is a variant of the model developed in Refs. 1 and 2. To account for the existence of the measurement tools in the wellbore, step changes in the wellbore radius have been incorporated into the model. The annular flow between the measurement tool and the well surface has been approximated with the hydraulic-diameter concept. The derivation of the analytical expression is similar to that in Refs. 1 and 2 and is not repeated here. We present only the final form of the expression and explain the terms involved.
The pressure drop between any point along the horizontal well (x) and the heel of the well (x = 0) is given by
in which ?=the density of the single-phase liquid, rwH=the hydraulic radius of the wellbore, and qh = the flux entering the wellbore from the reservoir. At a given point x, the flux (qh) and flow rate (qhc) are related by
in which L=the length of the horizontal well. Therefore, the sum of the fluxes along the length of the well is equal to the production rate [i.e., q=qhc(0)]. In Eq. 1, f is the local friction factor (fanning), and 0 denotes the friction factor at the heel of the well.
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