In order to validate a kick control method and a pressure loss model for Slim Hole, experiments on a pilot well were performed.
For the validation of the pressure loss model, flow parameters were varied so as to investigate all possible flow regimes.
For the kick control method, different drilling situations have been simulated. The paper underlines recommendations and operating instructions for the field.
Introduction
In Slim Hole Drilling, reduction of annular geometry will have very important impact or equivalent circulating densities (ECD) and kick developments. In order to understand the phenomena involved, experiments were conducted under actual well conditions on a Slim Hole pilot well (900 m deep, 4" casing and several pressure sensors at different depths).
The obvious advantages of using a Slim Hole pilot well are that all parameters of the experiments can be varied and monitored. This is the only mean to simulate an influx and allow the validation of models and rules as this cannot be done on an industrial well.
Early field experience and theoretical calculations have shown that annular flow is an important issue in Slim Hole drilling. As annular clearance is reduced, ECD can be generated even at moderate flow rates. In order to control the well and to ensure efficient hole cleaning while avoiding excessive pressure against the formation, it is necessary to determine the basic parameters that govern annular pressure losses (APL).
It is now well established that APL are very sensitive to drillstring eccentricity and rotation (RPM). Theoretical work at IFP that was supported by laboratory experiments and analysis of data in the literature has shown that the way these parameters influence APL depends on flow regime. The important point is that both eccentricity and RPM will modify the borders of the flow regimes as well as correlations along each regime.
The objectives of the hydraulic experiments performed in this project were:–to determine the evolution of pressure losses with flow rate and RPM under field conditions,–to vary flow parameters in a wide range so as to investigate all possible flow regimes,–to validate predictions of the IFP model in each flow regime.
A kick in a Slim Hole well can be more critical than in a conventional well due to limited annular volume. With 2.2. l/m capacity in annulus, a kick of 5001 would fill up 227m of hole as compared to 33 m in 8 1/2" hole with 6 1/2" drill collars.
In addition the APL induced by circulation of mud and rotation of the drill string affect the bottom hole pressure and therefore the gas influx rate and also create potential kicks which are less critical in conventional wells.
However kick detection is more sensitive on a slim hole rig because of limited volume of mud in circulation which allows to reduce the active pit area. The pit level sensor is then more accurate regarding the volume variations.
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