Coiled-Tubing Gyro MWD Improves Kickoff Direction Accuracy for Cased-Hole CTD Re-Entry Drilling

Abstract

Coiled tubing drilling (CTD) is an economical method of re-entering existing cased wells in mature fields to exploit bypassed reserves because of its versatility in accommodating a variety of candidates and its technological edge in drilling more efficiently. In re-entry drilling, kicking off with the right bearing attenuates the single biggest issue when drilling long laterals---weight transfer problems. Correct re-entry wellbore placement from vertically cased wells has traditionally required many single-function trips. The magnetic interference from the steel casing makes the acquired magnetic tool face measurements from traditional instruments inaccurate. Verification of the kickoff direction is usually performed with a wireline multi-axis gyro instrument. This process can be time-consuming and still requires additional correctional runs or directional changes prior to beginning drilling in the desired direction toward the target. Hence, the entire process of intervention, from setting a whipstock in the right orientation to drilling in the least tortuous direction from these vertically cased wells, requires a more efficient approach if CTD is to be a viable option in marginal projects. This paper assesses the limitations in the current methods of kicking off and details an innovative approach to improve re-entry wellbore quality through the use of an integrated coiled tubing gyro measurement-while-drilling (MWD) instrument. Introduction The majority of re-entry candidates are existing wells in mature fields, a significant percentage of which are onshore and vertical. These wells are re-entered either to extract reserves from compartmentalized, thin zones or to recover bypassed hydrocarbons. In vertical wells, magnetic tool face measurement from the conventional directional tool is used for orienting the bit in the direction of desired kickoff. The challenge, however, is to be able to kick off in the right bearing from vertically cased wells where magnetic interference from steel casing results in inaccurate magnetic tool face measurements acquired from traditional MWD instruments. Because the drilling is done in a blind condition, there is an increased risk of kicking off in an undesired bearing. Traditionally, the kick-off assembly for the build section consists of a downhole drilling motor with its initial adjustable kick off (AKO) set to give a reasonable amount of curvature to land the well horizontally in the target payzone. In most cases where the initial kick-off direction is not accurate due to unreliable magnetic tool face reading caused by proximity to the cased vertical well, direction correctional changes have to be made to the deviated well in the desired target direction. However, such directional adjustments can lead to increased friction loss because of the undesired localized doglegs in the wellbore. Wellbore directional nonconformity has a significant effect on the single biggest issue when drilling laterals: smooth weight transfer to the bit. Failure to adequately correct direction increases the risk of reducing overall potential payzone footage. Wellpath corrections required to compensate for the initial deviation initiate further problems in landing the well horizontally in the desired payzone because a significant component of the BHA's build-up rate capacity is consumed in just making turn adjustments. Therefore, the potential risk for creating a wellbore with impassable localized doglegs for completions is significantly increased. Since the kick-off assembly has a higher AKO setting on the motor in the build section than in the lateral section, directional adjustments in the build section are undesired as localized doglegs can compromise wellbore stability.

Advances in Coiled Tubing Re-entry Access Bypassed Reserves on Alaska's North Slope

“…In vertical cased wells, the desired kick-off direction for performing a re-entry is challenging since the magnetic TF transmitted by the tool is influenced heavily by the metal in the casing. 9 The need for economical re-entry of a large number of mature oil fields led to the development of a BHA upgrade to make possible gyro measurement while drilling or kicking-off with CTDD. Consequently, no additional trips out of hole are needed for a wireline gyro and correction of a deviated kickoff, and reduced tortuousity facilitates longer reach laterals, improving the economic viability of using CTDD for re-entry projects.…”