Dynamic Underbalanced Drilling technique

Abstract

Overbalanced drilling (OBD) is known to be cheap and simple but formation damage and reduced rate of penetration (ROP) are some of its disadvantages. However, underbalanced drilling (UBD) minimizes formation damage and achieve high ROP, but it is expensive, complex and can't be applied in many cases.

This application introduces a new technique that incorporates the advantages and avoids the disadvantages of these drilling systems and is referred to as Dynamic Underbalanced Drilling (DUBD).

In DUBD a pressure drop at the environ of the bit, below and around, is created that is restored to normal pressure above the bit and such conditions requires some minor modifications to the design of drill bit. Thus at the zone located below and around the bit underbalance conditions are dominated while the rest of the hole is overbalanced.

The anticipated cost of this technique is nearly zero and it is expected to save much of drilling costs. DUBD is expected to provide higher ROP that may exceed ordinary UBD in some cases, so saves time, reduce formation damage, saves costs (cheaper than OBD) and enables gathering information about the reservoir while drilling. This can save some of logging and testing costs. In addition, DUBD may reduce drilling problems, particularly time sensitive problems, and overcome problems of UBD. Furthermore, it can be used safely in over pressurized shale and salt formations, where UBD is not recommended.

Claims

1: The Dynamic Underbalanced Drilling (DUBD) technique is based on the following modifications on drill bit: 1—Modifying nozzle orientation: The nozzles are oriented in a direction so that the flow is parallel to the formation or don't hit formation and so keep the fluid velocity high in this zone. Two designs are given in the drawings. There are too many designs that can do the same target but one was mentioned for explanation. 2—Modifying nozzle size The size of the nozzle is selected to provide the maximum suitable fluid velocity for the well or to reach a selected fluid speed to create the required pressure drop as in the table in the explanation part. 3—Modifying bit profile To Keep fluid velocity around and below the bit high. Flow area around the bit must be decreased with increasing number of blades for example or width of blades of the bit so the velocity is kept high. Also to provide smooth path for the fluid to flow parallel to the formation being drilled. These modifications will according to general energy equation or Bernoulli equation create a pressure drop.

2: According to claim 1: These modifications are made to keep drilling fluid velocity high below and around the bit to create an underbalanced zone there to achieve the benefits mentioned below: 1—Increase ROP 2—Decrease drilling cost and time 3—Decrease formation damage 4—Get earlier data about the reservoir 5—Reduce drilling problems and drilling fluid losses 6—Provide better hole cleaning. As mentioned in the explanation part: The increase in ROP (as discussed earlier) is very great and is due to the created dynamic underbalance below and around the bit. As ROP increases the time required for drilling will decrease and thus the cost will be decreased also. Because formation will be subjected to the drilling fluid for shorter period of time so damage will be reduced greatly. The technique allows getting information about reservoir while drilling. These data include (permeability, reservoir pressure, fluid types and distribution, . . . ) as mentioned in the explanation Reducing open hole time will reduce drilling problems because most of them are time dependent. They increase if open hole time increases. Also the total fluid lost to the formation will be lower due to lower time for these losses. This technique improves hole cleaning as was discussed earlier.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0077] FIG. 1: As shown in the figure placing the nozzles in the center (white circle) of the bit and rotating them 90 degree (points radial outward i.e. the nozzle axis parallel to the arrows) so that the fluid will flow around the bit profile as shown by the blue arrows. This is one possible PDC bit designs that matches the DUBD requirements.

[0078] FIG. 2: As shown in the above figure, a possible design for DUBD in roller cone bits can utilize the extended bit nozzle to provide the required flow path. The nozzles are directed so the fluid will flow as shown by the blue arrows.

[0079] The two figures show two designs to be applied in DUBD. There are too many designs to do so and can be used in DUBD.