Well Control Analyses on Extended Reach and Multilateral Trajectories
Authors: Choe Jonggeun , Schubert Jerome J. , Juvkam-Wold Hans C. 
Published: October 2008

For economic and technical reasons, the industry has utilized dire ctional, extended reach, horizontal, and multilateral wells. For the planning of the well trajectory in a two dimensional plane, in the past we have used composite buildup charts or several directional equations depending on the trajectory type. We can develop a simple equation for the length and angle of the first hold section, by computing the hold length first using vertical depth, kick off point, horizontal departure, and buildup rates.

Surface choke pressures are highly dependent on kick vertical height in the well and well trajectory. As the final hold angle of the well increases from vertical, shutin casing pressure reduces to shutin drill pipe pressure and becomes the same as long as the kick stays in the horizontal section.

For the equal vertical depths, the maximum choke pressure is not sensitive to hold angles or hold lengths as long as the initial pit volume gain is the same. However, for directional and extended reach wells, choke pressure can increase even without kick expansion due to the change of kick vertical height in the build section. This may mask choke pressure reduction when the kick passes the casing shoe, especially if the kick off point and casing shoe depth are close.

In multilateral wells with more than one flowing wellbore, shut in drill pipe pressure is affected by the maximum value due to the kick influx volume and formation overpressure of each branch wellbore. Therefore, we should be careful in the interpretation of the shutin pressures. For safe circulation of the kick out of the hole, we must consider imposing additional pressure to compensate for hydrostatic pressure reduction resulting from kick migration and expansion from branch wellbores.

INTRODUCTION

For economic and technical reasons, the industry has utilized directional, extended reach, and horizontal wells. Several wells drilled from one platform for offshore development will reduce the development cost significantly and make the installation of production equipment efficient. These wells are also applicable for areas that will not permit vertical wells due to inaccessible obstructions or environmental concerns. Multiple targets can be reached by a single directional trajectory.

From the production point of view, horizontal or near horizontal wells are used to enhance recovery efficiency by increasing reservoir exposure or by intersecting fractured reservoirs. It can also reduce chances of water coning, especially for a thin reservoir. They have been used extensively in heavy oil reservoirs. In the early 1990's, the industry employed multilateral wells from which multiple wellbores are drilled from one main wellbore (also called the "parent" wellbore). Multilateral laterals have the same kind of advantages aforementioned. However, implementation will be much more complex than directional wells with a single wellbore.

Although technologies are well developed for these well sand there are numerous successes in the last decade,1,2 these wells still have high level of risk on drilling, completion, pressure imbalance and communication, and production management. It will require multidisplinary teamwork, throughout planning, and implementation as planned. Well control is one of the relatively unanswered questions. Well control is one of the most important operations, because improper well control followed by a blowout is the most expensive and feared operational hazard. Therefore, we must develop a firm well control plan for practical applications of these wells.

A kick is defined as an unscheduled flow of formation fluids into a wellbore. Well control may cover several activities for kick prevention, kick dete