Live Pressure Control Risk Analysis for Drilling Through Multiple Reservoirs
Authors: Karlsen Hans Christian , Nilsen Thomas 
Published: March 2015

In 2010, a risk analysis taking into account both uncertainties in reservoir parameters and at the same time accounting for real time pore pressure measurements during a drilling operation on the Norwegian continental shelf (NCS), was completed. The plan was to drill through seven reservoirs, each independent of each other with a vast amount of uncertainties in the pore and fracturing pressure due to years of injection and production.

The risk analysis was first made as an input to the drill plan, using best reservoir parameters available as well as expert judgments. Simulations were performed within a Monte Carlo (MC) framework. Results like for instance probabilities for fracturing and kick for drilling through the next reservoir(s) or tripping out, as well as an optimum mud weight for reducing risk, were obtained. The analysis was updated real time as the pore pressures were logged during the drilling phase, providing the best possible risk picture and giving the decision takers a possibility to update the drill plan before drilling through the next reservoirs.

This paper describes how the main characteristics of the reservoirs were modeled and how the risk analyses were performed. Simulations obtained, both before and during the drilling operations, are presented. Results like the probabilities of fracturing and kicks as a function of reservoir parameters and number of reservoirs drilled, as well as optimum mud weight for risk reduction, are emphasized.

Introduction

Risk analyses are widely used by operators on the Norwegian continental shelf (NCS) to manage risk during drilling and completion phases. The analyses should reflect potential hazards and unwanted events during the operations, and are mainly based on pre-assumptions and best available information. Uncertainties of well specific parameters are usually dealt with in a conservative manner. However, risk analyses taking into account real time pore pressure measurements have, to the author's knowledge, not previously been performed.

Considerable geologic uncertainty introducing potential narrow pressure margins were key issues in the planning of drilling a sidestep in a mature oil and gas field on the NCS. The field has been produced for some time, and several reservoirs had been subject to injection. Forthcoming decisions related to selection of mud properties and operational procedures were a challenge, and the overall risk level of the drilling operation was unclear. It was originally decided to undertake a quantitative risk analysis (QRA) to improve the overall risk picture and to form a stronger basis for decision-making in the development of the drilling program. Later in the project, one clearly saw the advantage of having the risk picture re-evaluated as the pore pressures were measured. The objective of this paper is to present the stages of the QRAs in the well planning process, the results and their applicability as decision support for the project.

The QRAs was performed by use of the KickRisk analysis tool. This tool is based on a pressure control model, where factors influencing the bottom hole pressure (BHP) throughout the various phases of the construction of a well section are decomposed. The model includes down hole, fluid, equipment and operator-related aspects and their impact on hydrostatic, friction and surge/swab pressure effects. The analysis implies gathering of input on the uncertainty of these quantities in terms of probabilities and probability distributions. By Monte Carlo simulation, the resulting probabilities of entering a state of underbalance with the pore pressure (kick) and of exceeding of the fracturing pressure (fracturing) are derived, and the factors that are main contributors to the risk are identified.