W.J. Lee

Using artificial neural nets to identify the well-test interpretation model

The authors thank Yeung et al. for their discussion about their original paper. They agree with Yeung et al. that their proposed scaling method, when applied to patterns with distinct subparts such as the one shown, represents an improvement on the method they proposed. This is particularly true because Yeung et al.s method eliminates the need to train the artificial neural networks (ANNs) on different sizes (scales) of the same pattern of a specific interpretation model. This paper presents the following comments for discussion and suggestions for further improvement.

New pressure transient analysis model for dual-porosity reservoirs

This paper presents a new analytical model for interpreting pressure-transient tests for wells producing from dual-porosity reservoirs. This model includes unsteady-state matrix flow and incorporates the effects of wellbore storage, skin, and, for gas reservoirs, desorption. The model is applicable to bounded and infinite-acting reservoirs.

Characterization of Tight Reservoirs

This paper gives methods to characterize tight gas reservoirs in sufficient detail to allow an engineer to make accurate long-range production forecasts. These forecasts are the bases for sound engineering and business decisions. Because of the complexity and variability of tight gas reservoirs, the authors can present only general procedures for developing reservoir descriptions. Accordingly, the authors illustrate a reservoir characterization method with three examples of successful tight gas reservoir studies. The procedures in these examples can be modified as needed for other specific formations or areas.

Characterizing average permeability in oil and gas formations

This paper reports that permeability in a formation frequently follows a unimodal probability distribution. In many formations, particularly sedimentary ones, the permeability distribution is similar to the log-normal distribution. Theoretical considerations, field cases, and a reservoir simulation example show that the median, rather than the arithmetic mean, is the appropriate measure of central tendency or average value of the permeability distribution in a formation. Use of the correct estimate of average permeability is of particular importance in the classification of tight gas formations under statues in the 1978 Natural Gas Policy Act (NGPA).

Rate Performance of a Layered Reservoir With Unsteady-State Interlayer Crossflow

This paper presents an analytical solution for rate performance of a well intercepting a bounded two-layer reservoir. The solution is for modeling two-layer reservoirs with or without crossflow, with each layer having different layer skins and drainage radii. The authors present the effects of partial completion and of different layer skins and drainage radii on the rate behavior in layered reservoirs.

Pressure behavior of layered and dual-porosity reservoirs in the presence of wellbore effects

This paper presents a new analytical solution for pressure-transient from layered reservoirs with or without crossflow. The analytical solution is for modeling layered-reservoir systems with unsteady-state or pseudosteady interlayer crossflow, commingled or stratified flow, and even dual-porosity systems with pseudosteady matrix-to-fracture transfer in the presence of skin, wellbore storage, and phase segregation.

Effect of Mud Filtrate Invasion on Apparent Productivity in Drillstem Tests in Low-Permeability Gas Formations

This paper shows that mud-filtrate invasion prior to drill stem testing in low-permeability gas formations can cause significant reduction of gas flow rate observed during the test. In addition, apparent gas permeabilities determined from test data can be much lower than the true values. These conclusions are supported by simulator results and field data.

Identification of Productive Layers in Low-Permeability Gas Wells

This paper presents new guidelines for determining net pay thickness in low-permeability, multilayered gas wells. These criteria were developed from a sensitivity study performed with an analytical solution for complex multilayered reservoirs. The purpose of this study is to determine whether many layers now considered to contribute to net pay actually have transmissibilities too low for the layer to be productive, causing performance projections from current singly-layer descriptive models to be too optimistic.

A new method for analyzing well tests with inadequate early-time data

This paper presents a new technique, the cumulative-curvature method, for analyzing well tests with such limited early-time data that only nonunique type-curve matches are possible. The method is applied to buildup tests and single-rate drawdown tests that are distorted by wellbore storage and are run in infinite-acting reservoirs. The method was successfully applied to 16 of 22 field examples, two of which are presented here.

Analysis of prestimulation test data in Devonian shale reservoirs

Prestimulation testing on Devonian shales has proved to be complicated because of the long duration of wellbore storage distortion that must be exceeded during flow and shut-in periods to obtain an analyzable test. This paper presents field examples of four prestimulation well tests that illustrate the practical difficulties in obtaining analyzable data. The authors were not able to analyze these tests with conventional semilog and type-curve methods. To understand the cause of the difficulties, they simulated a prestimulation well test to verify that the actual wells had not been produced long enough to exceed wellbore storage distortion. The authors then history-matched the actual test data from two wells using a finite-difference simulator. Because they lacked good estimates of many formation properties, they performed a sensitivity study that indicated that the calculated permeability-thickness product is only slightly affected by errors in porosity and net pay estimates. The authors conclude with guidelines to aid in the design and implementation of a prestimulation test in Devonian shales.