Build-up Construction in Salt Formations
Authors: Amaro Renato , Goncalves Clemente Jose de Castro , de Lucena Lira Humberto , Falcao Jose Luiz , Lomba Rosana , Modesto Alcofra Elisa 
Published: June 2014

The history of directional drilling in salt formation has shown that oil companies have a conservative approach regarding the limits of dog-legs (DL). Most companies recommend projects with DL at about 2 degrees/30 m for the build-up construction in directional wells. In order to increase the expertise on making a build up section in such formations, a project was developed to test these limits.

To evaluate the performance of the tool, a variable build-up trajectory was designed, as well as a criterion for the classification of trajectory was established. The plan starts with a DL = 1º/30 m and progressively seeks maximum tool deflection capability, around 6º/30 m, being the whole construction of the build-up in 12 ¼-in. salt section, mainly carnallite, with halite and tachydrite interchanging intervals.

As a final result, it is expected to get the optimized configuration values for the tool deflector, a drilling service company 9 ½-in. rotary steerable, and to obtain lateral, axial and torsional vibration data for each range of build section drilled. Consequently, value is added to the learning process in drilling salt formations, focusing the directional drilling aspects, and allowing confident movement towards challenging environments.

In summary, the main objective of this project is to test the construction of a build-up section in salt, gathering lessons learned to apply in the directional wells in the deep water pre-salt area in Santos Basin, throughout the development phase.

Introduction

In order to elucidate a lot of questions regarding potential problems in salt formations drilling, some suitable scenarios for drilling investigative wells were identified and a first scenario was selected, composed by layers of three different types of salt. It was an onshore field discovered in 1966 and with 20 wells drilled to date. A pilot vertical well was drilled with four main objectives:to core the total salt interval to provide samples for geomechanical lab tests;to identify the top of the salt layer;to perform hydraulic frac tests in order to get in-situ stresses andto run a complete set of loggings to characterize and to obtain sonic, elastic and density data from the salts.

Afterwards, a cement plug was held and the vertical well was sidetracked to a directional trajectory with a build-up rate ranging from 1 up to 6 degrees for 30 m. After drilling, a salt creeping test was performed by reducing the mud weight and verifying the well diameter reduction in response to the mud weight decrease. The obtained data will be used to calibrate the in house developed computational models used in studies of creeping and well design.

From the lessons learned in this first onshore well, a second well project was held with the objective of assessing the performance of steering tools in salt, now in ultra deepwater scenario. This project was an extension of an offshore exploratory directional well. The plan was to extend the exploratory drilling of a 37 degrees of inclination well, doing a build-up with dog-leg of 3.7º/30 m inside the salt (halite and anhydrite), reaching the base of salt with 85 degrees of inclination, following by a hold and, then, a drop-off in carbonates. Moreover, the drilling of the two mentioned wells intended to verify the tool performance regarding rate of penetration, geosteering and to evaluate expected operational problems in salt formations.

Temperature values were assumed for the top and bottom of the salt formation based on offset wells, with the purpose of evaluating well stability and mud weight requirements. There is a significant difference between the two scenarios covered by this paper. The thickness of the salt layer in the onshore well is about 250 m, similar to the one in the ultra deepwater well However, the overburden is different. Although both are buried by approximately 1.000 m of sediments, there is a water column of 2000 m in the offshore well.