Automatic Prediction of Downhole Pressure Surges in Tripping Operations
Authors: Gjerstad Kristian , Sui Dan , Bjorkevoll Knut S. , Time Rune W. 
Published: May 2016

Abstract
A simplified dynamic model of the tripping operation is used together with an ensemble Kalman filter to predict transient pressure surges when running the drillstring in or out of the hole. Dynamic downhole pressure measurements from a tripping operation with mud circulation are used as input to the Kalman filter. Such data can be achieved by mud pulse telemetry at the field just before the tripping operation starts. The model is automatically adapted to the particular situation (well, bit-depths, drilling mud, etc.). This is important since exact values of some downhole parameters, like viscosity of the drilling mud, might be unknown and/or changing with time. We show by comparison with filed measurements, that the automatically updated model is capable of reproducing the transient pressure surges in consecutive runs of the string without mud circulation.

Introduction
One of the most common reasons for well problems like inflow/kick, loss of circulation or fracturing is the surge and swab pressures during tripping operations. The axial motion of the string in these operations generates pressure oscillations that might violate the pressure margins in the well if the tripping speed is too high. On the other hand, running the string at unnecessary low speeds is time consuming and extends this non-productive time.
On traditional rigs, downhole measurements are unavailable during tripping. Predicting and controlling the surge and swab pressure must then be based on some sort of estimation. In this work we apply a medium-order dynamic model, and show that unknown and/or slowly changing parameters can be estimated from a limited amount of data which can be achieved in the field by circulating drilling mud while running the string just before the tripping operations start. This results in a set of updated model parameters which can be used in consecutive tripping operations. We show by comparison with field data from a North Sea well, that the parameters obtained automatically during such mud circulation gives an updated model that reproduces the transient pressure surges in tripping without circulation at various depths.
The requirement is that the drillstring is run while pumping fluid for a period of time before the tripping operations, so that the traditional mud pulse telemetry can be used to get dynamic pressure measurements by the BHA. If real-time measurements are available during tripping (wired drill pipe), the models capability to predict future responses could be utilized to optimize the tripping speed even further.
The basis for the model, which consists of 11 ordinary differential equations (ODEs), is presented in detail in [1]. However, some modifications to facilitate for automatic parameter estimation, is presented in this paper. It is a dynamic model that is designed for predicting the transient pressure surges in tripping operations.
It is assumed that the drilling mud can be represented as a Herschel-Bulkley (HB) fluid [2]. However, due to the complexity in the analytical solutions for laminar non-Newtonian fluids in pipes and annuli [3–5], some new simplified flow equations are applied. These simplifications are continuous functions of the bulk flow rate, and they give the frictional pressure gradient explicitly [1]. The result is a robust model that can be coupled with automatic parameter estimation techniques.