Abstract

The objective of this paper is to summarize the future application, limitations, advantages and disadvantages of UBD technology, based on eleven wells that were drilled in this type of reservoir.

The geographic location of the wells is the Center of the Neuquen Basin, located in the state of Neuquen, Argentina.

The objective was to define the reliable range of the stratum flow capacity value or permeability, in order to forecast the reservour productivity with UBD Technology, assuming the absence of formation damage (skin).

The method used was based on the transient-state production equation theory. Implementation was based on the drilling hydraulics design: maintaining constant Pwd close to the reservoir of interest formation pressure; stopping drilling to allow the development of transient production; establishing drilling conditions and detecting the production contributions in new stratums to be evaluated; determining the average production; correlating detected inflows with the pressures obtained by formation testing.

The method, together with other data, allowed us to estimate the structure potential productivity, despite the poor petrophisycs characteristics (Tight gas reservoirs) and to demonstrate that the results during completion could give reliabie properties estimatations used in the forecast.

Introduction

Almost all the wells involved in this study are located in the center of the Neuquen basin, where the sedimentary developement is thicker and more complete.

The highest interest of the project lies on the Grupo Cuyo levels. This unit is composed by fluvial sediments which turn to tidal up to offshore with modest reservoir conditions. The petrophysical conditions of this unit are controlled principally by the presence of abundant cement and matrix with associated diagenetic processes. Matching with it overlies the interval Loteniano-Chacayano which is composed by a pelitic lower section and an upper one of hard grey mudstone-type limestone, which due to its fissures or alteration, offers fair reservoir conditions. Next there is the column showing the evaporites of the Auquilco formation (Fig. 1).

Underneath lays the Precuyana column as sediment/filling material. This formation starts with an 1800 m thickness of igneous rock which has been described as a series of lava flows and piroclastic flow deposits which towards the top grade alternatively with arcilites and fall tuffs. In general the igneous rock composition is acid, with a major relative participation of piroclastic flow deposits further in the top position and close streams and flows in the lower position. These have been evaluated during UBD application in one of the examples considered in this study.

Based on loggings evaluation a sonic porosity between 5 and 8 % was calculated for the Grupo Cuyo formation. The permeability well testing data showed values between 0,03 and 0,1 mD.

The search for gas accumulation in these deep levels of the Cuyano sedimentary cycle, has led to the drilling of different exploratory wells for over 30 years. In most cases the wells confirmed the existence of gas, but it was necessary to find out whether the lack of profitable production from the economic point of view was due to its poor petrophysical conditions or to the application of inadequate drilling procedures that decreased the fluids production to the wellbore during its completion.

The potential skin damage was associated to high density drilling barite muds that were oftenly used in these reservoirs that have very low permeability and an a potential degree that produced secondary porosities.

Apart from the side-tracked in existing wells, new wells were drilled using the UBD methodology, their location can be seen in Fig. 2.

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