To increase oil and gas production deviated wells are drilled where the wellpath changes both in the horizontal and vertical planes. Directional change tolerances are becoming tighter because wells have to be placed with increased accuracy in soft overburden shales and within the reservoirs. Therefore there is a need for planning tools which can predict directional change capabilities of directional tools. Formations which are looked upon as easily steerable from a practical drilling point of view enables the directional driller to obtain rapid directional changes. While less steerable formations gives less directional responds and it is sometimes physically impossible to follow the planned wellpath. The goal of this study was to develop a method to evaluate the possible directional change responses based on formation rock strength. Drilling, log and rock mechanical data was collected in different offshore wells together with the actual directional changes obtained for a different rotary steerable system active steering settings. By correlating the unconfined rock strength, drilling and survey data trends between directional changes of the steerable rotary tools and the formation strengths were seen.
1. INTRODUCTION
To increase oil and gas production wells are drilled directionally where the wellpath changes both horizontally (azimuth) and vertically (inclination). Directional wells are generally the preferred solution when developing petroleum fields, especially offshore, where wells have to be drilled from the same template below the (future) production platform or sub-sea production unit. It is critical to place wells correctly to reach a reservoir target and to obtain the desired reservoir section length. When the reservoir target tolerances gets tighter it is even more important that the actual well path follows the planned path. In addition to following the planned well path, the uncertainty in the geological prognosis makes it sometimes necessary to adjust the well path in real time based on geological information collected while drilling, referred to as geosteering. Therefore there is a clear need for accurate predictive planning tools that can pre-estimate possible directional changes through different formations. A rotary steerable system (RSS) or a positive displacement mud motor is generally used to obtain the directional changes. The rotary steerable system use hydraulic power from the drilling mud to activate the steering pads which create directional change. Rotary steerable systems are one of the main technologies in use to change well directions today. The available RSS use pads to generate side wall contact with the formation to create directional changes at the bit. The directional change is controlled by directing the mudflow through the tool. The effect is controlled by the amount of controlled flow given in %, typically in steps on 20%, defined as % active steering setting. Because the systems rely on contact with the borehole wall to get directional control hole stability problems such as washouts, key seats and breakouts can negatively impact the directional performance on these systems [1]. Drilling experience with RSS in directional wells has also shown that some geological formations give less directional response. Some formations therefore need more active steering to obtain the same directional change.
