Measured Depth and True Vertical Depth in Oil and Gas Well Drilling

Introduction

Within oil and gas drilling, depth determination is important for wellbore planning, reservoir evaluation and drilling activities. Two commonly used depth measurements in the industry are measured depth (MD) and true vertical depth (TVD). Knowledge of these depth measurements is critical for improving well construction and production techniques. So lets dig into the differences in measured depth vs true vertical depth to better understand what they mean in the oil and gas sector.

Measured Depth (MD): The Actual Distance Traveled

Definition:

Measured depth (MD) is length of wellbore measured along actual path from surface to a target in subsurface formations. It is usually represented as the distance in feet or meter and indicates the total distance drilled from the drilling rig to a specified depth point in the wellbore.

Characteristics:

MD is a real and simple number that represents the actual distance drilled by the drill bit.

It allows for deviations, inclinations and horizontal drilling in the wellbore trajectory.

The MD measurement can only ever be greater than or equal to the TVD, due to the way wells exist in 3D space.

Applications:

Predictive fatigue data can be utilized to maximize drilling efficiency: – Leveraged for well planning and trajectory design;

Assists with drilling parameter calculations, such as, drill string length, casing requirements, and wellbore tortuosity.

Essential for drilling cost calculation and maximizing the operation of rig.

− Total distance drilled, which is used in directional drilling and horizontal well completions

True Vertical Depth (TVD): the “Downward” distance below the surface

Definition:

True vertical depth (TVD) is defined as the distance between a point measured down the wellbore and a reference bum on the surface, which is typically the elevation of the **drilling rig floor. It is the wellbore depth assumed **as if you drilled straight down vertically from the surface down to the target depth.

Calculation:

Trigonometric methods: TVD is computed by employing the **inclination and azimuth measurements of the wellbore acquired using downhole survey equipment.

By adjusting for deviation angles, TVD gives an accurate representation of subsurface formation depth relative to the surface.

Significance:

Reservoir evaluation: TVD is essential for reservoir evaluation; it shows the actual depth of hydrocarbon-bearing formations.

Applied in formation correlation and hydrostatic pressure calculation.

Crucial for testing pressure gradients and for forecasting pore pressure and fracture gradients.

– Assist in hydraulic calculations, such as bottom hole pressure (BHP) estimation.

Why you should have a clear understanding of MD and TVD in the oil and gas industry

1. Wellbore Placement and Trajectory Optimization

MD and TVD are well measurements that is still a critical design process, particularly for directional and horizontal wells (DHW). This depth information is critical to engineers in order to be able to drill more efficiently and effectively.

2. Reservoir Assessment and Formation Targeting

For geologists and reservoir engineers. it is critical in calculating the depth of productive formations. This assists in determining where to drill perforation zones, where to install production tubing, and how to formulate completion strategies to optimize hydrocarbon recovery.

3. Weighted Drilling Fluid And Pressure Control

TVD is pivotal in calculating hydrostatic pressure and mud weight requirements. Improper calculations of TVD can cause kick situations, well control problems and formation damage.

4. Well Integrity and Casing Design

Performance of casing and tubing strings is based on TVD, which offers formation pressures that help maintain well integrity. MD is also used to calculate total casing length needed for wellbore stability.

5. Production Execution and Economic Viability

Indeed, accurate depth measurements play an important role in production forecasting, economic analysis, field development planning, and more. Wells having more MD than TVD would lead to high costs because of extra complexity in the drilling.

MD and TVD in various well scenarios

1. Vertical Wells

MD and TVD are similar in vertical wells** where there is minimal deviation

Vertical Depth (TVD) → vertical distance defined by the perpendicular from surface to the well path. If the well follows perfectly straight path, than TVD = MD. Example: on 3000 meters drilled vertically, MD = 3000m & TVD = 3000m.

2. Directional Wells

In deviated wells MD > TVD because of the non-vertical trajectory.

For example a well has the measured depth of 3500m and a TVD of 3000m then it has 500m of deviation.

3. Horizontal Wells

MD is greatly higher than TVD in horizontal wells.

a well having a Measured Depth of 5000m, may have a TVD of just 2500m, identifying a long horizontal leg.

Conclusion

Remember, measured depth (MD) and true vertical depth (TVD) are crucial depth measurements utilized in oil and gas drilling operations. MD is the actual distance drilled along the wellbore path, and TVD gives a vertical depth reference to the surface. Recognizing how they differ is essential for proper wellbore placement, reservoir assessment, drilling refinement, and pressure management in the ever-changing energy market. MD and TVD have become invaluable tools for oil and gas professionals looking to improve well construction and maximize hydrocarbon recovery while maintaining safe and efficient drilling operations.