Formation Evaluation In High Angle And Horizontal Wells - A New And Practical Workflow

Logging measurements are axially focused and generally deep reading. In near vertical boreholes, measurement volumes are approximately parallel to formation layering (Fig. 1, upper panel). 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 (Fig. 1, lower panel). 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.