Short Radius Reentry Drilling Dramatically Increases Well Production in Venezuela
Authors: Mollinedo Miguel , Hill David , Ingold Charles , Salazar Julio , Stracke Mark 
Published: December 2007

Abstract
An extremely long lateral (1933 ft) drilled from a short radius curve (85 ft radius to horizontal) at 10,975 ft is now on-line at 2000 BOPD. This production rate is more than 6 times the performance of the vertical well that was reentered. The cost of the reentry was only 33% more than the cost of the original vertical well.
The economic success of the well is the result of careful application of short radius technology to a very deep reentry candidate. The process of selecting the candidate, a directional plan, the short radius drilling system, an oil-based drilling mud, and the completion are reviewed. Changes to the original plan made both before and while drilling, are highlighted, as well as the use of several new variations of the short radius drilling tools.

Introduction
The Alturitas Field is located in Venezuela, approximately 120 miles 55W of Maracaibo, Venezuela (Fig. 1). The field was originally discovered in 1950 and the Marcelina reservoir has produced over 19 million barrels of oil with a total Alturitas field production over 50 million barrels of oil. The Alturitas No. 22 vertical wellbore was originally completed in a lower overpressured zone and had been previously recompleted in the Upper Marcelina, a Paleocene age formation.
The Upper Marcelina is a 152 ft gross sand thickness sandstone reservoir rock containing a principal production interval 82 ft thick. The overlaying formation to the Upper Marcelina is a 50 ft. coal formation (Fig. 2).
The Alturitas No. 22 completion in the Upper Marcelina has a bottom hole pressure of 4815 psi at 11,000' while the bottom hole temperature is 240 F. Oil production from the Alturitas No. 22 vertical well was 300 BOPD of 22 API gravity oil.
The operator's goal for this reentry project was twofold. First to determine if a reentry horizontal penetration through the Upper Marcelina formation would increase well productivity as they anticipated and as had estimated through candidate recognition modeling. And second to determine if short radius drilling technology was capable of drilling the required well path without penetrating the sensitive overlying coal formation. Both the economic success and technical success of this program depended on the ability to drill the reentry sidetracked wellbore entirely in the productive interval of the Upper Marcelina formation.
Previous attempts to drill high angle and horizontal wellbores in the Upper Marcelina had seen limited success. The inert nature of the coal to mud chemistry stabilization and its friability and lack of competency created extremely difficult hole stability problems for directional drilling applications.

Reentry and Drilling Challenges
Target Formation Thickness. The 82 ft thickness of the targeted section of the Upper Marcelina and the inability to drill any of the reentry sidetrack wellbore through the overlying coal formation were the design criteria that dictated a short radius well profile. The overlying coal formation has historically caused drilling difficulties. The soft and highly friable nature of the coal formation prevents the use of drill string stabilizers when drilling through it and has a susceptibility to formation collapse. Additionally, a "hardline" maximum TVD was specified to minimize drilling potentially reactive shales below the target formation. Several options for sidetracking and building at least some of the angle above the coal formation were evaluated. All were rejected because: P. 793^