Engineered Casing Cementing Programs Using Turbulent Flow Techniques

Abstract

Effectively cementing production casing to isolate productive zones is essential to the successful completion of oil or gas wells. Obtaining adequate primary cement jobs still pose some difficult problems in many areas. Although remedial cementing methods are available, their expense adds greatly to well completion costs. Drilling fluids contain additives which are contaminants to oil well cements and thus effective mud removal from the annulus is essential. Cement additives have recently been developed which permit the attainment of turbulent flow in the annulus at reasonable pump rates and thus permit more effective mud removal. This paper presents field experience on a 26-well program where turbulent flow techniques were applied in primary cementing. Through this program a new factor to consider in primary cementing has come to light. This factor is contact time, which is defined as the period of time that a particular point in the annular space remains in contact with a cement slurry being displaced in turbulent flow. Careful control of pumping rates and slurry volumes is now being maintained in order to give specific contact times. Significant correlations have been made between long contact times and improved primary cementing.

Introduction

The problem of effectively cementing production casing to isolate productive zones and eliminate undesirable fluids is still one of the most perplexing problems facing the industry. In the past, there has been a tendency to live with the primary cementing problem and utilize corrective techniques such as block squeezing to facilitate isolation. Operators in some areas, where primary cementing is difficult, have made it a policy to block squeeze every well. Increasing costs have dictated the need for reducing completion costs and, logically, the need for improved primary cementing techniques evolved. To further complicate the problem, drilling muds in use today contain materials which are contaminants to oil well cements. However, the use of these muds is necessary for other reasons, and thus the need for effective removal of the contaminating mud from the annular space is now more pronounced than before.

A review of earlier cementing methods in several southwest Louisiana fields notorious for primary cementing failures, revealed that expensive remedial cementing techniques were often necessary. Of 20 wells completed at inland marine locations in the Hackberry fields, eight wells or 40 per cent required block squeezing for isolation. The cost of this remedial cementing varied from approximately $6,000 to $40,000/well, depending on the severity of the problem, with the average being approximately $15,000/well.

With the above in mind, a program was initiated in mid-1962 to evaluate various primary cementing methods. Since effective mud removal from the annulus is necessary to the attainment of a suitable primary cement job, concentration was placed on turbulent flow techniques. This emphasis was made because the benefits of turbulent flow techniques in accomplishing effective mud removal have been known for some time; however, practical limitations in equipment and cement slurries have precluded its application. Recent developments toward improvement in primary cementing techniques have now made it possible to obtain turbulent flow in the field within practical limits. In addition to turbulent flow displacement rates, an attempt was made to evaluate the effects of the use of tailor-made cement slurries, special centralizers, pipe reciprocation and pipe cleaning. All of these techniques were designed to contribute to the effectiveness of the turbulent flow concept of mud removal. At the outset, a detailed review of previous primary cementing techniques in the area failed to reveal a simple solution to the problem. This was due, in part, to a lack of recorded data. Therefore, special emphasis was placed upon detailed pre-engineering of each job, close on-the-job control, and the recording of all pertinent data. As a part of the pre-engineering phase, the rheological properties of the turbulent flow slurries were measured in the laboratory in order to determine the displacement rate required for turbulent flow in the field. So that each job could be judged in retrospect, a very important part of the on-the-job control and data recording phases of the project was the field measurement of the rheological properties of the slurries after mixing and before being pumped into the wells.

JPT

P. 503^