Abstract

This paper discusses the development and field application of a single well, three-phase, numerical, thermal simulation model which represents the coupled wellbore and reservoir mechanics following closure of the annulus steam production, or "casing blow" in producing wells. The goal of the model is to provide an understanding of the physical mechanisms associated with casing blow shut-in which may cause a drop in oil production, and hence, lead to operational procedures which would minimize this lost production. The simulation model accurately represents the exact geometry and dimensions of the wellbore tubing, annulus, and casing. Both counter-current multiphase, annulus flow, and coupled, crossflow effects between the wellbore and reservoir are modeled. Limited entry perfs and wellbore damage effects may also be included. Stable and robust controllers permit true pumped-off conditions to be modeled, as well as multiple casing blow cycles.

The model was used to predict the magnitude and duration of oil production loss for a Kern River field pilot following casing blow shut-in. The prediction exactly matched field observations, including changes in the annulus pressure. Prior to casing blow shut-in, oil production is achieved by both gravity drainage and inter-well viscous drag forces. Depending on near well conditions, a variety of responses can be expected following shut-in. Preshut-in wellhead pressure is a key indicator of well response. Wells that were mostly impacted by shut-in generally do not recover without remediation Following shut-in, there is a transient period during which the oil rate transitions form its original decline onto a shallower, gravity drainage curve. As long as the casing blow steam production is shut-in, the drop in oil production seen at shut-in does not return to preshut-in levels, again, without remediation The mechanistic insights gained from the simulation model resulted in successful field strategies to improve or ameliorate the lost oil production following casing blow shut-in. These strategies will be discussed.

Introduction

A pilot was initiated at Texaco's Kern River field steamflood operations in November, 1994 to ascertain the impact on oil production of shutting in the annulus steam vapor production from producing wells. This pilot was preparatory to full-field casing blow shut-in. A single well simulation model (SWM) was developed with the following main objectives:

  1. To provide an interpretation of the pilot observations in terms of physical mechanisms in the wellbore and the reservoir.

  2. To design and recommend operational procedures to minimize the oil production loss resulting in the wellbore and the reservoir.

  3. To extrapolate the pilot interpretation to a full field scenario, with recommendations for future field and simulation work.

Even though the model was developed specifically for Kern River, the methodology is general, and should permit applications to other problems. Consequently, this discussion will be as generic as possible with regard to the model formulations.

Model Capabilities

To be of practical value in the current context, the SWM must be capable of accurately modeling the following physical wellbore and near wellbore phenomena:

  • exact geometry and dimensions of the tubing, annulus, and casing,

  • exact physical properties of the wellbore components,

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