Abstract

A horizontal well was planned for a location in Maracaibo Lake, Venezuela, and it was required to assess its stability. A pilot vertical hole was drilled including an oriented core. Several laboratory and field techniques such as an elastic strain relaxation (ASR), differential strain analysis (DSA), acoustic anisotopy in the cores, breakouts, sonic logs and focal mechanisms were used to estimate the in-situ stress field. A cased-hole minifrac was available from a nearby well in the reservoir sand. While finishing the drilling of the pilot hole, running logs and cement-plugging, laboratory static and dynamic tests were performed for both, cap and reservoir sandstones. Linear elastic theory and Mohr-Coulomb failure criteria were used for the borehole stability analysis. Information from the rig site about mud weight used in unstable sections of the pilot was used to calibrate the linear elastic model and bound the maximum horizontal stress component. As a result of the analysis, safe mud weight limits to prevents collapse and lost of circulation were released to the drillers for both, the inclined and horizontal sections. The well was drilled without any stability problems. In this case it was shown that simple linear elastic models, when calibrated with field data, give reliable, first approximation values, specially useful when real-drilling-time answers are needed.

The stability during production was studied using a 2-D finite element model with a generalized plasticity constitutive equation. A safe drawdown to prevent failure was determined. The well was completed open hole without any liner. It is producing above the original potential without any sanding or stability problems.

Introduction

Horizontal well activities in Venezuela started around the 80's and now the companies have aggressive horizontal drilling programs. Even though there have been successful cases, there have been some horizontal well failures, as well. The problems have been related to geological and petrophysical uncertainties, formation damage in mature reservoirs and collapses due to borehole instability. Programs considering vertical pilot holes with oriented coring have been developed to increase the chances of success in reservoirs in which very little information has been gathered. This approach has allowed to make timely evaluation of the reservoir rock and in situ stresses before the horizontal hole is drilled and therefore minimize risks from geomechanical and mud drilling design points of view.

An example of such case was the planning of a horizontal well in a reservoir in the Maracaibo Lake area, Venezuela. This was the first horizontal well in this field and it was drilled with the purpose of draining the atic oil accumulated against an adjacent normal fault. The well was placed sub-parallel to the normal fault (with strike N36W approximately) as shown in Figure 1. The horizontal well was planned to be approximately parallel to the fault.

It was difficult to asses the geomechanical risk for a horizontal well in this area without having an idea of the rock strength and the in situ stress field. Moreover, the regional stress environment in the Maracaibo Lake area is very complicated and it was not known if the horizontal well was going to be drilled in the most risky direction. Trying to decrease as much as possible the uncertainties, a vertical pilot hole with 60 feet oriented coring was drilled. Additionally, three hundred feet conventional coring was recovered for sedimentological studies. The pilot-hole and horizontal well plan is presented in Figure 2. Sonic logs were added to the conventional petrophysical logging program to asses a strength profile for the rock along the pilot hole.

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