Formation Evaluation in High Angle and Horizontal Wells – A New and Practical Workflow
by David Maggs, Schlumberger
presented at 11:00 on the 5 th of December, 2012.
Presented at SPWLA 2012, 53rd Annual Logging Symposium
Logging measurements are axially focused and generally deep reading. In near vertical boreholes, measurement volumes are approximately parallel to formation layering. In this environment, logging measurements provide optimal vertical resolution and information about formation properties beyond the mudinvaded zone.
Logging-while-drilling (LWD) measurements are predominantly acquired in high angle and horizontal (HaHz) wells. In this environment, measurement volumes are approximately perpendicular to formation layering and deepreading measurements may respond to multiple layers, creating complications for subsequent interpretation. For many years LWD measurements were considered unsuitable for quantitative petrophysical evaluation, however the problem was not so much with the measurements but with the interpreters’ assumption that measurements respond to a single layer, as had been their experience with vertical wells.
With an increasing proportion of HaHz wells drilled for field development, a workflow that compensates for geometrical effects is required to determine true formation properties from the logs. A new workflow has been developed to address these issues and is now available in commercial software. Starting with the acquired logs, a layered earth model of the structural geometry proximal to the wellbore is created. Log responses are used to identify boundary intersections with the well trajectory. In the case of multiple crossings of a single boundary, the user simply “joins the dots” to define the formation geometry. Formation dips extracted from LWD images are plotted on the layered earth model to define the relative dip between the wellbore and layering. Formation geometry from other sources can also be imported to guide the structural interpretation.
Once the approximate geometry is defined, initial estimates of the formation properties (such as gamma ray, horizontal resistivity, vertical resistivity, bulk density and neutron porosity) for each layer are obtained from the measured logs. When the well crosses a layer more than once, the user can chose to either enforce layer property consistency or allow lateral property variation. Fast forward models of the LWD measurements are then used to compute tool responses based on the geometry and formation properties defined by the formation model. If the computed logs do not match the measured logs, the geometry and/or formation properties are manually but easily updated until an acceptable agreement is achieved. Upon completion, the final model is a validated representation of both the subsurface geometry and formation properties.
The resulting layer properties, which have been derived from the measured logs by correcting for formation geometry around the well, are then available for use in conventional formation evaluation techniques. Field examples demonstrate the significance of geometrical effects to LWD measurements in HaHz wells, and the capabilities of the workflow to enhance formation evaluation in this environment.
David Maggs, CV
David Maggs is currently LWD Petrophysics Domain Champion for Techlog, based in Grabels, France. He joined the company in 1988 and worked as a Wireline Field Engineer for 8 years in South America and the North Sea. He has since worked in Management and Technical positions in a variety of geographical locations including North America, Europe, South America, the Far and Middle East, mainly supporting LWD measurements and Well Placement operations. David holds a Master’s Degree in Mechanical Engineering from the University of Southampton, England.