Abstract

Newly developed software has illustrated that the safe mudweight window to avoid borehole collapse and fracturing can be obtained from the direct use of drilling and/or the use of sonic data.

Instantaneous or post analysis using inverted drilling operational data like WOB, RPM, flowrate, and mud properties can be done by applying an inverted ROP model to predict rock compressive strength. This rock compressive strength is then used in conjunction with survey, in-situ stresses and rock failure criteria to predict the safe mud weight window. The sonic logs are also used to obtain the rock compressional properties through published correlations. The rock compressive strength is then used the same way as the drilling data approach.

This paper gives a solid field example from a North Sea well where both methods are applied and compared. The field case provides proof that the new software is a very good tool for predicting a safe mud weight window to avoid borehole collapse and fracturing while drilling.

Introduction

During drilling operations it is crucial to maintain the hydrostatic mud weight pressure between the fracture and collapse pressure at any depth to ensure trouble free drilling. The casing program is also dictated mainly from the fracture and collapse gradient. Therefore, the knowledge and use of the correct fracture and collapse gradients can save the operating companies large sums of money. When estimating the safe operational mud weight window two important parameters are

  1. the strength of the rock, and

  2. the stress conditions the rock is subjected to.

The question is; how can these parameters be monitored while drilling a well. Today there is no way of measuring compressive rock strength or stresses while drilling. The best method of determining compressive rock strength is by triaxial testing of core samples, but the problem is that these data will then only be available after the well is completed. These triaxial data are then used in conjunction with logs through correlations when planning the next well.

This paper describes a decision support system for well planning, and follow-up of well integrity (safe operating mud weight window) during drilling operations. The system, is a Windows based modular system, using Internet as the communication network for data exchange and technological support.

One module in the system is the inverted ROP module which back calculates the compressive strength of the formation while drilling. This in-situ compressive strength will be used dynamically to calibrate the in-situ stresses which in conjunction with knowledge of the overburden and pore pressure, and the wellbore survey dictates the collapse and fracture pressure for the wellbore instantaneously. If the well has already been drilled and the above information is available, the true vertical depth collapse and fracture gradient can be predicted as part of the post analysis or for pre-planning the next well in the area.

Another approach to obtain the collapse and fracture gradient is from log data, where the rock compressive strength of the rock is estimated from the sonic travel times. After the compressive strengths as a function of depth are estimated the same approach as for the use of drilling data to obtain in-situ stresses is applied.

Using such a decision support software system, data may also be more readily available for display instantaneously or for use in future planning, thus providing a more complete set of data for design prognosis. P. 253

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