Since 1975, Petrobras has worked with Brazilian Portland cement manufacturers to develop high-quality Class G cements. The Petrobras R&D Center has analyzed each batch of Class G cement manufactured by prequalified producers to API Spec. 10 standards and to Brazilian Assoc. of prequalified producers to API Spec. 10 standards and to Brazilian Assoc. of Technical Standards (ABNT) NBR 9831 standards. As a consequence, the Drilling Dept. at Petrobras now is supplied by three approved Class G cement factories strategically located in Brazil. This paper statistically analyzes test results on the basis of physical parameters of these Class G cements over 3 years. Statistical indices are reported to evaluate dispersion of the physical properties to obtain a reliability index for each Class G cement.
After manufacturing the first Brazilian Class G cement for nearly 15 years, until 1988, Petrobras, cement service companies, and cement manufacturers had developed uniform batch analysis and approval procedures.
On the basis of the cement manufacturers' experience and the results of batch approval tests, the Drilling Dept. Cementing Sector at Petrobras decided to modify this cement systematically. The Cementing Sector began to analyze and approve alternative batches from two highly reliable cement manufacturers: batches from a third manufacturer were maintained in this same manner. This procedure was adopted with the goal of analyzing only random batches, from these three manufacturers as the quality and reliability of their cements increased.
This newly adopted approach was based strictly on the experience and intuition of technicians involved in Class G cement analysis and development, however, and lacked a technical basis. Thus, this paper was written not only to test our theories, but also to improve our knowledge of the quality of three Brazilian Class G cements.
In 1975, Petrobras began a procedure that culminated in the technological qualification of Class G oilwell cement for the Brazilian drilling industry. Obtaining a good Class G cement, uniform not only among batches but also within each batch, is not easy. It requires accurate technology involving many variables, such as rigorous selection and strict dosing of raw material, velocity and temperature control at the rotary furnace, and clinker crushing criteria.
For cements to reach the present quality level, which has been maintained since 1984, both Petrobras and suppliers invested heavily in the development of human resources. Thereafter, the Petrobras Drilling Dept. and Cementing Sector with the participation of cement service companies, cement manufacturers, and the Brazilian Assoc. of Portland Cement (ABCP) began the Brazilian Standards of Cement for Oil Wells project to create quality standards for Class G cement. ABNT approved these standards, and Petrobras adopted them in March 1988.Petrobras adopted them in March 1988.
To answer our questions regarding cement quality, we decided to use a mathematical treatment reflecting the behavior of Class G cements. Data were selected on the basis of the standardized physical properties of the three cements Petrobras uses.
We chose to conduct a sampling as soon as possible (assuming that the manufacturers had a well-controlled cement production process) to provide a considerable data population that would make true statistical analysis possible. May 1986 through May 1989 was chosen as the data collection possible. May 1986 through May 1989 was chosen as the data collection period on the basis of Class G cement evaluation test results filed with period on the basis of Class G cement evaluation test results filed with the Petrobras Drilling Dept.