3. SUPPLEMENTAL BALANCE SHEET INFORMATION

Inventories

The increase in inventory was partially due to acquired inventory of $78.0 million related to our 2022 acquisitions (see “Note 4 - Business Combinations and Asset Acquisition”). The remaining increase is related to our increased activity levels in 2022.

Property, Plant, and Equipment, Net

The increase in net property, plant, and equipment was primarily due to assets of $945.7 million acquired through our 2022 acquisitions (see “Note 4 - Business Combinations and Asset Acquisition”). This increase was partially offset by $48.2 million of FTSI assets sold in the sale lease-back transaction immediately following the FTSI acquisition.

Depreciation and amortization expense related to property, plant, and equipment for the years ended on December 31, 2022, 2021 and 2020, was $261.2 million, $139.9 million and $150.6 million, respectively. Depletion expense for the years ended on December 31, 2022, 2021, and 2020, was $0.5 million, $0.2 million, and $0.1 million, respectively.

Intangible Assets, Net

Amortization expense related to intangible assets was $5.6 million, $0.6 million, and none for the years ended December 31, 2022, 2021 and 2020, respectively. The electric frac licenses were transferred to property, plant and equipment in connection with the construction of our electric fleets in 2022.

The estimated future amortization expense related to intangible assets is $34.0 million in 2023, $34.0 million in 2024, $33.7 million in 2025, $32.1 million in 2026, $12.1 million in 2027, and $57.1 million thereafter.

Accrued Expenses