Development of Software To Predict Mud Weight For Pre-Salt Drilling Zones Using Machine Learning
Authors: Pereira Luis Guilherme , Gandelman Roni Abensur , Clemente Ricardo Gomes , Teixeira Pedro Henriques Santos , Teixeira Gleber T. 
Published: June 2014

Drilling salt zones, especially in deep and ultra-deep waters, presents several challenges to be overcome. One of the main problems faced by drilling engineers while designing and executing a section in salt zones, is the formation flow to the wellbore. Temperature and pressure exerted by the rocks above the salt formation make it have a plastic behavior that tend to flow, and close the wellbore caliper as the bit drills it. The deeper the salt formation, the higher the temperature and pressure above it and, consequently, the higher the tendency to flow to the well, causing several problems, such as high torque, drag and stuckpipes. In order to avoid these problems and to keep the wellbore stable, the drilling fluid must exert a minimum hydrostatic pressure on the formation. Thus, the mud weight is a very important parameter that needs to be determined with a good precision. Since there are no reliable tools to help drilling designers to predict the mud weight based on the wellbore information and its lithology, this is a very difficult task. Nowadays, the prediction of mud weights for pre-salt wells is made based on the designer experience and it's updated during the drilling job as the engineers analyze the wellbore behavior. The lack of precision on estimating the correct mud weight causes several operational problems, currently related to high nonproductive times. Petrobras is developing software to predict adequate mud weight in order to avoid salt fluency based on the lithology of the well and a big historic of wells drilled in pre-salt zones. Initial tests show that the software is able to predict the mud weight with a very good precision. This article shows the development of the software and initial results obtained.

Introduction

Pre-salt drilling in Brazil coast presents several challenges to be overcome. The high water depth leads to very narrow operational windows and new technologies (such as MPD, CCD, new drilling fluid systems, etc) are frequently required so as to make the drilling job reliable. Due to the high hole depths, special rigs are also required, making the drilling costs higher than the ones for wells in other regions. Logistics may also be an issue, since the number of rigs and boats in the region has not grown as fast as the number of operations.

However, one of the main problems faced by drilling engineers while drilling salt zones is the tendency of the salt to flow to the well. In downhole conditions, temperature and high pressure exerted on the salt formations make them present a plastic behavior. Thus, as the bit drills salt zones, they tend to flow to the region with lower pressure (the well) reducing the wellbore diameter and, frequently, leading to stuckpipes. This is a common and serious problem while drilling pre-salt wells, as several hours (and sometimes, days) are spent in attempts to liberate the drillstring, increasing the non productive time and costs. In some cases, the whole well is lost.

The salt flow tendency depends on several factors such as the type, the presence of interlayers in salt formation (different types of salt or igneous rocks, for instance), depth, temperature and pressure. The higher the pressure and temperature exerted on the salt formation, the higher the flow tendency. Different types of salt also present different tendencies to flow. Pure anhydrite, for instance, presents a very low flow tendency and usually does not pose a problem for drilling engineers. However, anhydrite with interlayers of other more fluent salts may pose several challenges during drilling job. It was also observed that salt formation with interlayers of igneous rocks present a very high tendency to flow posing serious problems to drilling engineers.