Differential sticking is known to be influenced by drilling fluid properties and other parameters, such as the characteristics of rock formations. In the past, multivariate statistical analysis techniques and simulated sticking testes using different drilling fluids have been performed to identify and modify parameters that lead to differential pipe sticking in order to minimize or prevent sticking. Recently, an application of neural network methodology to predict differential pipe sticking incidents in Gulf of Mexico has been published by Halliburton 1. This paper presents two different types of artificial neural network that can provide solutions for problems associated with differential pipe sticking.
A stuck pipe database was developed with data from 64 side tracked and horizontal wells drilled in reservoir section using oil based and synthetic drilling fluid from different fields in the Persian Gulf. Two three-layers feed forward networks; Multi Layer Perceptron (MLP) and Radial Basis Functions (RBF) with back propagation training algorithm were used to develop stuck pipe predictive models for oil base and synthetic drilling fluid together. Using these models, prediction of the probability of stuck pipe may be undertaken to monitor drilling operations for stuck pipe avoidance. A sensitivity analysis was also done by applying data of different fields separately to identify the parameters that had more effect on tendency to differential pipe sticking. The proposed methodology can be used for optimum drilling fluid design during well development in Persian Gulf, Offshore Iran.
Introduction
Stuck pipe is a common problem known to cost the industry hundreds of millions of dollars around the world. Differential pipe sticking problem is a predominant threat to drilling engineers in drilling highly inclined, horizontal, multilaterals, and re-entry wells, especially in offshore operations 2–6.
The concept of differential pressure sticking of drill pipe was first reported by Helmick and Longley 7 in according to laboratory tests. They stated that pipe sticking results when the drill pipe becomes motionless against a permeable bed and a portion of the area of the pipe is isolated by filter cake. Typically, the occurrence of differential pressure sticking can be diagnosed when the drill pipe cannot be rotated or moved up or down, but unrestricted mud circulation is still possible. Although these symptoms are similar to Key Seat sticking, they usually occurs under different drilling conditions. Significant mud overbalance, as well as an exposed permeable section, must also exist for differential sticking to occur. What is clear is that as many reservoirs become depleted, a significant number of wells will be drilled with high overbalance pressures, thereby maintaining the industry's concerns over differential sticking.
The likelihood of differential sticking is increased further with the length of the permeable section that is open to the drilling fluid. The continued trend towards extended reach and horizontal drilling means that increasing lengths of permeable formations are exposed. Clearly, the nature of the rock formations encountered certainly cannot be altered. Therefore, if those formations carry a high risk of differential sticking, this has to be accepted. Also, high overbalance pressures may be unavoidable if they are needed to maintain well control or wellbore stability in other parts of the openhole section. However, mud composition and properties can be modified, within limits, in the prevention of differential sticking.
In the past multivariate statistical analysis techniques and simulated sticking testes using different drilling fluids have been performed to identify and modify parameters that lead to differential pipe sticking in order to prevent or minimize sticking. A review of published literature and laboratory data establishes the importance of mud filter cake properties (thickness, shear strength, and lubricity) on the differential sticking tendencies of muds.
