A General Framework to Describe Drilling Process States

Abstract

Automation and digitalization of drilling requires shared knowledge about the state of the drilling process: is the bit on-bottom drilling or is the driller making a connection; is the borehole in good condition or is it sloughing? Yet there is no shared, clear and usable definition of what a drilling process state is, nor an agreed method to calculate it. In this paper, we propose a method to clarify the concept of drilling process state. A set of partial differential equations, respecting boundary conditions, can describe drilling operations. The set of all possible discrete changes of boundary conditions, therefore, defines the set of all possible drilling process states. Equality or inequality of logical expressions of at most two boundary values characterizes a discrete change of a boundary condition. For instance, if forces applied to the bit by the formation are zero, this corresponds to an off-bottom condition, while forces greater than zero means that the bit is on-bottom. Such simple logical conditions are microstates, and an orthogonal set of microstates defines a drilling process state. An analysis of the drilling process from the perspective of these microstates defines an orthogonal basis of microstates. It is possible to define uniquely any drilling process state in this orthogonal basis. There are a finite number of possibilities to move from one state to a different state by changing only one single microstate, which leads to the construction of an implicit graph of possible states. In this implicit state graph, the change from one state to another state that corresponds to more than one modification of the microstates corresponds to a path in the graph. However, the microstate basis depends on the type of drilling process. The paper will provide examples of different microstate bases for conventional drilling, backpressure managed pressure drilling, and dual-gradient managed pressure drilling. Microstates also cover abnormal drilling conditions, such as hanging on a ledge, or flow obstruction in the annulus by a pack-off. They are, therefore, more powerful descriptors than "rig activity codes". The required fidelity of the drilling process state depends on its use, for example for controlling drilling equipment (process control), for calculating key performance indicators (process statistics), or for user feedback (human factors engineering). This work is part of the D-WIS initiative (Drilling and Wells Interoperability Standard). D-WIS is a cross-industry workgroup providing the industry with solutions facilitating interoperability of computer systems at the rig site. The definition of a microstate is a simple logical statement, easily implemented in computer software. The paper provides an example of a simple algorithm, which will enable others to leverage the work in the commercial, interoperable, environment.