Stability Analysis of Horizontal Wells in Orinoco Belt, Venezuela
Authors: Rodriguez H.A. , Silva R.J. , Zerpa L.B. , Fung L.S.K. , Camino M. 
Published: December 2007

One unconsolidated heavy oil reservoir from Venezuela's Orinoco Belt has more than one hundred wells, few of them being horizontal with high rate production. One of the horizontal well, has a completion problem and another one has sand production problems. The goal of this study is to asses the geomechanical stability conditions of these two wells and determine possible remedial strategies, as well as procedures to avoid similar problems in the future.

The first well was completed in the closest sand to the basement with an approximately horizontal drilled section of 2000 feet. The diameter of the wellbore is 1.4 times the slotted liner attempted to complete the well. However, it was only possible to complete half of the well's horizontal planned section length. Due to the fact that the sand is unconsolidated and the well was drilled along the maximum horizontal stress direction, borehole failure may has occurred after drilling, leading to an unsuccessful completion of the well over the planned length.

The second well was completed in an upper sand which is composed by fine grains, containing also high clay concentration of about 8-9 percent. The diameter of the wellbore is 1.75 times the slotted liner attempted to complete the well, and there was no problem completing it. Nevertheless, the well was plagued by sand production almost immediately after the start-up.

The behavior of these two wells aimed to analyze the geomechanical stability problems under drilling and production conditions.

Although the reservoir is unconsolidated, the simulation results indicate that drilling a horizontal well in this reservoir using an overbalance condition is generally stable.

To produce a failure condition for the first well, it is necessary to apply an instantaneous drawdown of 200 psi or higher, which is more severe than letting the well to reach this drawdown slowly. Hence, no borehole failure should have been expected since such drawdown has not yet been applied. Therefore the inability to complete the well over its entire length might be attributed to the borehole trajectory.

To reach a failure condition for the second well, the bottom hole pressure must be reduced instantaneously just a few psi below the reservoir pressure. Given the fact that higher drawdown are needed to maintain well profitability, then a pre-packed liner or finer mesh slotted liner should be considered to prevent sand production.

The differences in geomechanical behavior of the formation are due to the lower sand having higher stress resistance as well as larger grain size than the upper sand. It seems that openhole completion in the lower sand can be stable if the well is initially drawdowned slowly to avoid an abrupt pressure gradient development. Once the steady-state production is achieved, a 200 psi pressure drawdown can be sustained without sand production problems.

The stability of the horizontal wellbore during drilling is azimuth insensitive within the horizontal stress ratio measured for this formation.

Introduction

The reservoir has more than 100 wells with 20 of them being horizontal. Of these twenty wells, one had completion problems while another has presented sand production problems. The purpose of this work is to analyze geomechanical stability conditions for these wells and determine a possible remedial strategy as well as procedures for avoiding similar problems in the future.

This paper reflects the importance of the role that geomechanical reservoir simulation models can play in the optimization of field production practices, as well as in understanding important geomechanical effects on the observed reservoir response.

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