INTERPRETATION OF PRESSURE TRANSIENT TESTS OF HORIZONTAL WELLS WITH

Abstract

The oil and gas industry has long recognized the importance of understanding the behaviors and trends of pressure and fluid flow dynamics for damaged and stimulated horizontal wells. It has also been recognized that the existing theories to predict these behaviors and trends have not been effective due to the geologic factors, as well as drilling, completion, and production processes. Previous researches and studies over the last two decades have shown different types of factors such as the presence of multi-damaged zones, multi-segmented fractures, branching, asymmetry, and deviation from either the vertical direction or the horizontal direction of the wellbores as a result of in-situ stress distribution.

The main purpose of this study is to find new applications for the well test analysis rather than the classic applications that are focusing basically on the characterization of formation and determination of the permeability and skin factor. The new applications for the well test analysis are evaluating performance of the zonal isolations and hydraulic fractures and determining the locations of the isolations and fractures that do not perform as designed. Another objective is to investigate pressure behavior and flow regimes of a horizontal well containing either zonal isolations or hydraulic fractures.

The objectives in this study are achieved by using different analytical models. These models have been derived to simulate the pressure responses and flow regimes in the vicinity of the wellbore and the outer boundaries of the formations. Based on the new derived models, different analytical solutions and type-curve matching sets have been developed to characterize formations.

The first part of this study focuses on the impact of the zonal isolations on pressure behaviors and flow regimes of horizontal wells. Horizontal wells with multiple zonal isolations have become a common completion technique in the oil and gas industry. Sand and asphalt production problems, damaged zones and water cresting or gas coning are the main reasons for using isolators to sustain or improve oil and gas recovery. However, the use of such isolators introduces negative effects on the pressure behavior of horizontal wells.

This research introduces new analytical models for studying the effect of this completion technique on pressure behavior of wells with multiple isolated zones. These models have been derived based on the assumption that reservoirs can be divided into multi-subsequent segments of producing and non-producing intervals. Based on the pressure and pressure derivative, the models can be used to estimate the impact of isolators on the pressure behavior. The effects of the number and length of isolators have been investigated for wells having different lengths.

A set of type-curves of dimensionless pressure and pressure derivative versus dimensionless time have been generated for two cases. The first case is for wells in an infinite reservoir having short dimensionless wellbore length and multiple-isolated zones, while the second case concentration on very long wells in an infinite reservoir. These plots can be used to verify the number and length of zonal isolations originally installed, as well as to determine the number and locations of malfunctioning isolators. These plots can also be used to locate segments where sand is produced and intervals of water cresting or gas coning are present.

The main finding is that the pressure of these wells behaves similarly for all cases. The dominant effect of the isolators can be noticed only during the early time flow regimes, i.e. during the early radial or early linear. The behavior of the late time flow regimes, i.e. pseudo radial is not affected by the presence of isolators.

The second part of this study focuses on the pressure behavior and flow regimes that are developed for horizontal wells intersected by multiple-inclined hydraulic fractures. The fractures either fully or partially penetrate the formations. Horizontal wells with multiple hydraulic fractures have become a common occurrence in the oil and gas industry, especially in tight formations. Recent studies have shown that fractures are asymmetric, inclined with respect to the vertical direction and the axis of the wellbore, and partially penetrate the formation in many cases.

This study introduces new analytical models for interpreting the pressure behavior of horizontal wells with multiple hydraulic fractures. The hydraulic fractures in this model could be longitudinal or transverse, vertical or inclined, symmetrical or asymmetrical. The fractures propagate in isotropic or anisotropic formations. In addition, they have different dimensions and different spacing. These models can be solved to calculate various reservoir parameters, including directional permeability, fracture length, skin factors, angle of inclination and penetration ratio.

Type-curve matching technique has been applied using the plots of the pressure and pressure derivative curves. A set of type curves have been generated for the inclined transverse and longitudinal hydraulic fractures associated with horizontal wells having different inclination angles from the vertical and different penetration ratios.

Tiab's Direct Synthesis (TDS) technique has been applied also using the plots of the pressure and pressure derivative curves. Several unique features of the pressure and pressure derivative plots of both longitudinal and transverse fractures models were identified including the points of intersection of straight lines for different flow regimes. These points can be used to verify the results or to calculate unknown parameters. Equations associated with these features were derived and their usefulness was demonstrated in this study.